Gravitational waves are infinitesimal vibrations of spacetime, extremely weak and therefore very difficult to detect. Produced by extremely energetic events such as the merger of black holes or neutron stars, these ripples propagate at the speed of light and carry energy through space. After decades of attempts and technological advancements, the first signal of a gravitational wave — produced by the merger of two black holes over a billion light-years away from Earth — was observed in September 2015 by the LIGO-Virgo collaboration, thanks to the two twin LIGO interferometers (in the USA).

The discovery of gravitational waves, awarded the Nobel Prize in Physics in 2017, is one of the most significant achievements of modern physics, confirming a fundamental prediction of Albert Einstein‘s Theory of general relativity. This result has also opened a new window for observing the universe, giving scientists the opportunity to study cosmic phenomena that do not emit light or other forms of electromagnetic radiation, such as black holes.

To measure the signal of a gravitational wave, highly sophisticated and precise instruments are required: laser interferometers, such as those found in the LIGO and Virgo observatories. These are large, advanced infrastructures consisting of two long arms (4 kilometers at LIGO and 3 kilometers at Virgo) arranged perpendicularly to each other. Inside these arms, laser beams travel through ultra-high vacuum tubes, and thanks to highly advanced suspended mirrors at the ends of the arms, the beams are reflected multiple times to increase their path length before recombining, creating what is known as an “interference pattern”.

As a gravitational wave passes by, it distorts space, altering the length of the arms and thus the path traveled by the laser beams (one will be longer and the other shorter). As a result, the interference pattern also changes. This variation is extremely small, of much less than the diameter of an atom, but modern interferometers are sensitive enough to detect even such infinitesimal deformations.

The Einstein Telescope (ET) is a European project aimed at constructing a large facility to house a next-generation gravitational wave detector. Its goal is to study the universe through gravitational waves, tracing its history back in time, almost to the moment just after the Big Bang, in order to reconstruct how it formed, evolved, and to understand what its future could be. It is named so because it will be an instrument used to “observe” gravitational waves arriving on Earth from deep space, and it is dedicated to Albert Einstein, who first hypothesized their existence as a consequence of his Theory of general relativity.

The Einstein Telescope project has been included in the Roadmap of the European Strategy Forum on Research Infrastructures (ESFRI), the European body that provides advice on which crucial scientific infrastructure investments should be made in Europe.

Currently, there are three instruments in the world capable of detecting gravitational waves: the LIGO interferometers in the United States, Virgo in Italy, and KAGRA in Japan. These will continue to operate for over a decade before passing the baton to next-generation instruments like the Einstein Telescope (ET). ET will be able to detect gravitational waves with unprecedented sensitivity, observing a volume of the universe at least 1,000 times larger than that explored by current detectors, reaching/travel/coming back in time to just a few thousand years after the Big Bang.

Unlike LIGO and Virgo, which operate on the surface, the Einstein Telescope will be underground, significantly reducing seismic noise and other external interferences. Its arms will also be longer, ranging from 10 to 15 kilometers, compared to LIGO and Virgo’s 3-4 kilometers. Furthermore, ET will employ advanced technologies to reduce thermal noise and enhance low-frequency sensitivity. It will operate in a frequency range from 1-3 hertz up to 10 kilohertz, enabling the observation of events that are currently difficult to detect, such as the mergers of supermassive black holes and continuous gravitational waves from unknown sources. On the contrary, current interferometers are more sensitive to higher frequencies (from 10 hertz to 5 kilohertz), typically above 100 hertz, which allow for the detection of events like stellar black hole and neutron star mergers.

ET will study the universe through gravitational waves, for certain types of sources, up to cosmological distances. By observing the gravitational waves produced by the merger of black holes and neutron stars, as well as by other extreme astrophysical events, ET will be able to retrace the evolutionary history of the universe, taking a journey back in time toward the Big Bang. Not only that: thanks to ET, the observation of so-called “multimessenger” events will also become more frequent, where the detection of gravitational waves is associated with that of electromagnetic signals, allowing for the extraction of a vast amount of different types of information about the behavior of matter under extreme conditions. ET’s discoveries could also contribute to the study of some of the great mysteries of the universe, such as the nature of dark matter and dark energy, which together make up over 95% of the entire universe.

The Einstein Telescope will consist of a series of interferometers located in an underground tunnel at a depth of 100 to 300 meters. The scientific community is evaluating two possible configurations for the future instrument: one in the shape of a delta (Δ), with three arms of approximately 10 kilometers extending from the vertices of an equilateral triangle, and to be built on a single site; or an L-shaped configuration, with two perpendicular arms of about 15 kilometers, similar to current detectors. In this second case, two twin interferometers would be constructed and located in two distant distant sites.

In both configurations, a series of experimental caverns will house seismic isolation towers, large optical devices, laser systems, cryogenic systems, and vacuum systems, which require advanced electronic and mechanical technologies to ensure optimal performance in detecting gravitational waves.

ET must operate in conditions of absolute silence, far from external interferences. Seismic stability, in particular, is an essential factor to ensure the high performance of the gravitational interferometer, especially in the search for low-frequency oscillations. For this reason, it will be located underground, at a depth of between 100 and 300 meters, further shielding it from seismic and environmental noise, which will be significantly reduced compared to the surface.

The site selection for the Einstein Telescope is still ongoing. Currently, two locations are competing to host the interferometer: the Italian site in the former metalliferous mine area of Sos Enattos in Nuoro, Sardinia, the Euroregion Meuse-Rhine area at the border between the Netherlands, Belgium, and Germany, and a site in Lusatia, in the Saxony region (Germany).

The observatory must be built in an area far from both natural sources (seismic activity) and anthropogenic sources (vehicle traffic, industrial operations, transportation) that could mask the weak signal generated by the passage of a gravitational wave. Additionally, the geology of the host site must ensure minimal presence of groundwater to allow for the stable and safe construction of the underground environments that will constitute the ET laboratory. The final choice of the site (or sites) where the experiment will be installed is expected in 2027.

The hinterland region of Sardinia is an ideal location to host the Einstein Telescope. The area around the former Sos Enattos mine, the Sardinian candidate site for the infrastructure, is extremely stable from a seismic standpoint, a crucial factor for ensuring high performance of the interferometer. Additionally, the mine’s rock and surrounding area — mainly composed of granite — along with the minimal presence of groundwater, make the site particularly suitable for the safe construction of an underground laboratory. Lastly, the region of interest, spanning the municipalities of Bitti, Lula, and Onanì, is characterized by vast rural areas with very low population density, further enhancing the “quietness” of the environment, a necessary condition for the operation of the Einstein Telescope.

In the most optimistic scenario, the Einstein Telescope could begin its scientific operations around 2035. The exact timeline will depend on the progress of construction (which will take about 10 years) and funding, which will result from a combination of European funds, contributions from individual participating countries, and global scientific collaborations.

Italy plays a key role in the Einstein Telescope project, both due to its established experience in the field of gravitational wave detection, and as a possible site for the observatory. The Italian candidacy is supported by the Italian Government, the Ministry of University and Research (MUR), the Autonomous Region of Sardinia, and is scientifically coordinated by the INFN in collaboration with the National Institute for Astrophysics, the National Institute for Geophysics and Volcanology, and research institutions and universities across Italy.

The Italian candidacy is supported by the Italian Government, the Ministry of University and Research (MUR), the Autonomous Region of Sardinia, and is scientifically coordinated by the INFN in collaboration with the National Institute for Astrophysics, the National Institute for Geophysics and Volcanology, and research institutions and universities across Italy, and is also supported by projects funded by the NRRP:

ETIC (Einstein Telescope Infrastructure Consortium), initiated in 2023, has two main objectives: to conduct a preparatory study for the technical and economic feasibility of the ET observatory, and to create or enhance a national network of laboratories dedicated to developing the technologies necessary for the future interferometer at the INFN locations, universities, and research institutions involved in ET.

FABER/MEET, whose overall objectives are the improvement, technological upgrading and implementation of large scientific networks dedicated to Earth monitoring and observation. In particular, FABER aims to develop a seismic observatory at the former Sos Enattos mine to record currently unknown seismic signals.

TeRABIT will establish a high-performance network based on fiber optics to ensure rapid data transmission. This network will be crucial to support Sardinia‘s candidacy to host the Einstein Telescope, which will generate large amounts of data that need to be shared with a scientific community spread across the planet.

Italy has the largest gravitational wave scientific community and has the greatest expertise in Europe for the construction of gravitational wave detectors and the world’s largest expertise for the construction of underground scientific laboratories. Approximately one third of the ET collaboration is composed of scientists affiliated with Italian research institutions.  

Italy is also home to Virgo, the only European gravitational-wave observatory currently operational. Virgo has been conceived and built by Italian and French scientists and engineers and its upgraded version, Advanced Virgo is now a pan-European project. In addition, Italy has developed the world’s largest underground laboratory, the Laboratori Nazionali del Gran Sasso, demonstrating great expertise in the design, construction, and operation of large-scale underground research facilities. 

Because in any configuration ET is an exceptional tool of science. As stated in the executive summary of the so-called CoBA paper “All the triangular and 2L geometries that we have investigated can be the baseline for a superb third generation-detector, that will allow us to improve by orders of magnitudes compared to second generation-detectors, and allow us to penetrate deeply into unknown territories”. 

Within this framework, Sardinia is the best place to build ET – independently of the configuration adopted – since it maximizes its scientific return, per euro invested, considering its geophysics, geology (already widely discussed), environmental noise level, and latitude.  

Approval for the possible construction of the Einstein Telescope in Sardinia, for both proposed configurations, was also granted by the preliminary conference of services on the pre-feasibility study for ET, held in Cagliari on November 7. This confirmed the feasibility of building the observatory in Sardinia and thereby strengthening the candidacy of Sos Enattos, the first among the competing sites to obtain favorable technical opinions through the instrument of the preliminary conference of services. 

In the 2L configuration, the ET observatory consists of a network formed by two geographically separated L-shaped infrastructures. Obviously, a single L-shaped detector on its own is not a viable option for ET; instead, the scientific objectives of the observatory can only be achieved through the combined operation of the two L-shaped sites as a single, integrated observatory. This applies both from a scientific perspective and from a governance perspective.  In addition, the broader ET framework includes other complementary facilities, such as computing centers, innovation hubs, and theoretical research divisions. 

The international scientific community has demonstrated that the 2L configuration maximizes scientific return, accelerates the timeline, and minimizes risks in the integration, commissioning, operation, and upgrade phases. 

The 2L configuration generally provides a higher scientific return compared to the triangular option. Moreover, the 2L and triangular geometries show very similar performance across all parameters for both binary black holes and binary neutron stars merger events, except for luminosity distance (an astronomical measurement that allows us to calculate the distance to a celestial object by comparing its intrinsic brightness with the brightness as seen from Earth). In this case, the 2L geometry performs three times better than the triangular geometry. 

Yes, there are several potential strategic advantages associated with the 2L option: 

• It strengthens the pan-European dimension of the project by involving more countries and research centers;
• It minimizes construction risks, thanks to the reduced complexity of the infrastructure. In fact, in the 2L option each site hosts a single detector composed by two interferometers, whereas the triangular option requires a single site to host six nested interferometers forming three detectors;
• It lowers the financial burden on non-host countries, making the project more accessible to European nations with smaller budgets. Host countries are asked to cover most of the civil infrastructure costs, while other countries primarily contribute to the detectors. Since the 2L configuration involves four interferometers instead of six, the overall cost borne by the consortium is reduced.

The 2L design requires a partnership between one site in Southern Europe and one in Northern Europe. This requirement is driven by scientific reasons, related to gravitational waves’ signal localization capability. Sardinia has identified a partner in the Saxonian candidacy, which significantly strengthens the italian proposal. However, in principle, any site that meets the requirements in terms of environmental noise and rock quality, and is located at least 1000 km from Sardinia, could be considered as a suitable partner. In addition, countries participating in the Italian proposal will contribute to the ET network by hosting key ET facilities.

There will be no limitations due to the Einstein Telescope for activities other than industrial, infrastructure, or mining ones .In particular, there will be no restrictions on residential construction.

Yes, no restrictions of any kind are expected, except in areas designated for the research center or technical installations.

No specific restrictions or changes are expected.

There will be no issues for mechanical soil work related to agriculture or forest management. Mechanical activities related to mining or similar uses will be subject to case-by-case evaluation, in agreement with the relevant authorities and according to protocols that will be defined. Wells will be still usable for irrigation or other purposes.

Current studies foresee a 315,000-square-meter science campus, of which only 11,500 square meters will be allocated to buildings, with about 80% of the surface allocated to redeveloped green areas. Expropriations may be necessary only for portions of land affected by surface construction (e.g., research center, entrances to underground tunnels, access to vertical shafts, etc.), for which agreed and transparent solutions with landowners and local administrations will be sought.

In order to “allow the construction and full operation of the Einstein Telescope research infrastructure,” Decree Law of February 24, 2023, converted into Law No. 41 of April 21, 2023, stipultates for certain municipalities in the affected area listed in Annex 2, that for a range of activities considered “critical” for the operation of the infrastructure, any additional permits, however named, required for the conduct of economic activities, must be issued by the competent authorities in agreement with the Ministry of University and Research (MUR), after consulting the National Institute for Nuclear Physics (INFN).

The activities include: production of cement, lime, and plaster; manufacture of concrete, cement, and plaster products; cutting, shaping, and finishing of stone; electricity generation; construction of roads and railways. According to the law, future changes may occur regarding both the municipalities involved and the listed economic activities, “based on objective needs related to preserving the full functionality of the research infrastructure and reducing potential interference”.

Since the decree came into force, the only activities not authorized have been limited to wind farms.

The construction phase is estimated to last approximately 7–9 years, depending on whether the telescope is in an L-shaped or triangular configuration. However, the exact timeline will be defined in the detailed design phase, which follows the current one. The construction of the infrastructure complies with European, national, and regional regulation for reducing environmental impact. Specific design choices will aim to minimize the duration of works, as well as the number and size of construction sites.

The most impactful activities are expected to be excavation works. A key advantage of the Italian proposal is concentrating excavation activities in only one of the three vertices, reducing the impact to a limited area and the need to transport excavated materials: some of these materials may be reused directly on-site for constructing structural components of the underground tunnels. Furthermore, local roads and storage areas will also be designed to minimize disturbance.

From the earliest design stages, the ET scientific community has focused on solutions ensuring infrastructure efficiency and improving local services and accessibility, with strong attention to environmental and energy sustainability. The goal is to minimize negative impacts and promote sustainable resource use, landscape-friendly solutions, and low-impact infrastructure.

Additional interventions include improvements to local roads, reinforcement of the Minghetti dam near vertex V1, renewable energy systems, digital connectivity, and solar lighting. The Sos Enattos site will be enhanced as a research and cultural hub, including visitor facilities. Surface buildings and laboratories will be designed to blend into the landscape using sustainable construction solutions.

According to preliminary impact studies, based on the Conceptual Design Study funded by the European Commission under the Seventh Framework Programme:

During the construction phase, 65–75% of the impact will be at the regional and national levels and will primarily affect companies in the following sectors and their respective supply chains: construction, mechanical engineering, engineering and geological consulting firms, transportation, retail and wholesale, hospitality, and food service.

During the operational phase, according to the same studies, the regional impact will primarily involve companies in the sectors (and their respective supply chains) of hospitality, food service and catering, cleaning, retail and wholesale, security, infrastructure maintenance, technological systems and software, heating fuels, and electricity.

Furthermore, the activities taking place at ET will undoubtedly have a positive impact in terms of technology transfer on suppliers of goods and services and throughout the relevant value chain. This is because the machinery, instruments, hardware, and software required for each experiment conducted at the facility will necessarily be developed through constant interaction between researchers and suppliers, involving a continuous transfer of skills and knowledge.

Examples include:

• Improvement of local road networks and connections to the main road network through road infrastructure providing access to the site;
• Reuse of construction access roads to expand the trail network and create a cycling route network; development of solar-powered lighting systems;
• Improvement of digital connectivity infrastructure;
• Creation of tourist routes dedicated to exploring the observatory site and the surrounding area;
• Energy, environment, and landscape;
• Reuse of excavated material for natural hazard mitigation works, such as slope stabilization and the restoration of waterways, or for the technical and environmental backfilling of abandoned mining cavities;
• Development of innovative facilities for the treatment and reclamation of excavated material, to be adapted for local waste treatment;
• Innovation in the wind energy sector toward more efficient and controlled facilities, for example through vibration monitoring systems that improve plant efficiency;
• A push toward green energy systems;
• Surface-level buildings – whether for research or hospitality – designed to be energy-self-sufficient, promoting low-carbon-footprint construction;
• Architectural design of buildings and access points to underground infrastructure that enhances the landscape and utilizes local natural materials for construction (cladding and insulation);
• Promotion of local culture and the Sos Enattos site;
• Thanks in part to the future ET-SUnLab infrastructure, the Sos Enattos site will become a hub for research groups involved in the study, design, construction, and installation of ET, as well as hosting a visitor center;
• Development of the mining museum, featuring spaces dedicated to ET science and technology, highlighting the mine’s history and its transformation into a leading center for cutting-edge research;
• Renovation of hospitality facilities (guesthouse and restaurant);
• Enhancement of the site’s culture as a place for underground exploration, from metal mines and rare materials, such as blende and galena, to excavations for ET tunnels and caverns to study the universe and astrophysical events.

In addition to the more strictly material benefits, there is a further impact on society linked to scientific output, innovation and technology transfer, the growth of human capital, the scientific appeal of the site, and thus the dissemination of the knowledge produced by ET. Once fully operational, ET will attract a steady stream of young talent, PhD students, interns, and researchers who, during their time at the research facility, will acquire skills that will positively influence the rest of their professional careers.

Furthermore, meetings, conferences, and events, both for experts and the general publicm, as well as initiatives to promote scientific culture, in which informal interaction and the exchange of ideas are invaluable, will be able to take place near the ET site, contributing to high-quality tourism that is less dependent on seasonal fluctuations.

During the construction phase, for the triangular configuration, the total potential impact on employment, taking into account both direct and induced effects, is estimated to be equivalent to approximately 3500 full-time workers per year over the assumed 9-year construction period (2800 for the L-shaped configuration). Since construction requires the rotation of various professionals across the different construction phases, some specialists may work for only part of the year; therefore, the total number of people involved will exceed 3500/2800 per year.

During the operational phase, the potential impact on employment, taking into account both direct and induced effects, is estimated at approximately 1200 jobs per year (for the triangular configuration; 900 for the L-shaped configuration), excluding visiting researchers and ET employees. It should be noted that the model for European laboratories dedicated to ET is currently under study: it cannot be ruled out that the Sardinian site will be designated as a major research center, which would result in an increase in the numbers indicated here for the operational phase. Furthermore, the number will depend on the decision made at the European level regarding the detector geometry (a single installation at one site, or two installations at two different sites).

During the construction phase, the total value of annual transaction flows associated with the construction of the ET (equivalent to total turnover), calculated as the sum of direct and induced demand over the 9-year construction period, is estimated at approximately 5.8 billion euros (4.3 billion in the case of the L-shaped configuration).

During the operational phase, assuming for convenience an expected operational lifespan of the ET of 30 years, the value of the annual transaction flow linked to the operation of the ET is approximately €200 million (€150 million in the case of the L-shaped configuration), for a total present value of approximately €6 billion (€4.5 billion in the case of the L-shaped configuration).

Part of this flow will generate regional and local benefits in terms of socio-economic and territorial impact.

The “ET community” will need to be inhabited for varying lengths of time, with corresponding needs in terms of transportation, education and training, healthcare, and access to artistic, environmental, and cultural heritage. It is necessary to ensure an adequate level of transportation services and frequencies even outside the summer season, with connections between the site area and the rest of the island, which are essential to making it an attractive residential area overall, to accommodate new resident families, and to ensure a pleasant experience of the entire island.

Equally important is the presence of international schools—from preschool through middle and high school—in the area of interest to ET, as well as adequate connections with the international high schools already operating in Sardinia. Similarly, there must be clinics and emergency services of an adequate standard in the Infrastructure’s area of interest, as well as connections to the larger, specialized healthcare facilities on the island.

As for daily life in the area, recreational and community activities need to be strengthened, such as: movie theaters, theaters, sports facilities (swimming pools, gyms…), as well as parks, nature trails, and food and wine tours. The highlighted features serve as a draw for the ET community, but are also accessible to residents and tourists, creating a mutually beneficial situation and fostering overall growth in the area.

Furthermore, regardless of the numbers involved, the proponents do not plan to build housing specifically for ET staff or guests, with the possible exception of a small guesthouse within the research center for short visits. Consequently, the aim is to facilitate the integration of newcomers into the residential and environmental fabric of the resident population. Naturally, this must respect the ultimate freedom of choice of those who come to work in the area.

The municipalities in the Project Area are Lula, Bitti, Onanì, and Orune. The four municipalities have varying degrees of involvement, depending on the surface works that will impact their territories. The main construction site will be exclusively within the territory of Sos Enattos (Lula).

The municipalities are and will be fully involved in all territorial planning activities. A regional steering committee has been established with the aim of planning policies for accommodation, services, transportation, and industrial facilities from the ground up.