First cross-border underground connection in the Brenner Base Tunnel project. Representatives of the European Commission and institutional leaders from Italy and Austria attended the ceremony. Numerous guests celebrated the breakthrough in the exploratory tunnel of what will be the world's longest railway tunnel. 

Today, 18 September 2025, the breakthrough in the exploratory tunnel of the Brenner Base Tunnel marked a historic event 

Austria and Italy, together with the European Union, are celebrating a decisive step forward in one of Europe's most important railway projects for more sustainable, modern and future-oriented mobility. The ceremony was attended by numerous prominent figures, members of the Italian and Austrian supervisory boards of BBT SE, as well as representatives of construction companies from both countries, around 1,000 guests.

Breaking through in the exploratory tunnel, 1,400 metres below ground

Breaking through the diaphragm wall, approximately 1,400 metres below the Brenner Pass, is one of the most exciting moments in tunnel construction. At 2.30 p.m., European Commissioner Tzitzikostas, President Meloni, Chancellor Stocker and the two transport ministers pressed the start button together. 

There was great anticipation in lots H53 Pfons–Brenner (Austria) and H61 Mules 2-3 (Italy): while the miners and project teams waited underground, guests were able to follow the event live from a marquee set up at the Brenner Pass. Using a jackhammer, the miners broke through the rock wall separating the two excavation fronts. The meeting of the teams, accompanied by hugs and applause, highlighted the exceptional teamwork, technical innovation and international collaboration involved in the project.

The exploratory tunnel: one of a kind

At 57.5 km, the exploratory tunnel of the Brenner Base Tunnel is one of the longest of its kind in the world. It will not be used for rail traffic, but was built for geological surveys, internal transport, maintenance and also as an emergency escape route.

The extremely high precision of the measurements is particularly noteworthy: the section up to the Brenner Pass was excavated from two sides, from the Wolf and Mules access tunnels. A maximum tolerance of only 20 centimetres was set for the 29.1 km section, but the actual deviation was only six centimetres. Thanks to this accuracy, it was possible to construct the two main tunnels without having to resort to complex corrections.

This concept of a separate exploratory tunnel, which serves as a “precursor” to the Brenner Base Tunnel, is unique in the world in terms of length and precision and underlines the high technological complexity of the project.

The exploratory tunnel is located centrally beneath the two main tunnels, at a depth of approximately 10-12 metres.

The exploratory tunnel: data and historical notes

The exploratory tunnel follows the route of the two main east and west tunnels of the BBT, positioned at a depth of approximately 12 m below them: it provides information on the quality of the rock and allows the geological conditions for the excavation of the main tunnels to be studied. Once the tunnel is operational, the exploratory tunnel will be used for water drainage and maintenance of the facilities.

Three of the nine TBMs used in the BBT project (Clio, Gunther and Serena) were employed along the exploratory tunnel, excavating a total of approximately 42 km. The remaining kilometres were or are being constructed using traditional methods. The exploratory tunnel has a total length of 57.5 km, of which 27.2 km are in Italy and 30.3 km in Austria.

History

• 28/04/2008 Ceremony to mark the start of work on the Aica exploratory tunnel (Italy): 10.5 km, double shield TBM (6.3 m diameter)
• 04/12/2009 Start of excavation work in Austria: Sill Gorge exploratory tunnel: 5.4 km, 26m² excavation section, traditional advance
• 03/11/2010 Fall of the Aica-Mules exploratory tunnel diaphragm (Italy)
• 11/12/2013 Completion of the Sill Gorge exploratory tunnel (Austria)
• 26/09/2015 Start of excavation with TBM for the Ahrental-Pfons exploratory tunnel (Austria): 17 km, open TBM, 200 m long (8 m diameter)
• 05/01/2016 Start of work on the Wolf exploratory tunnel
• 29/03/2019 Groundbreaking ceremony for the ‘southern exploratory tunnel’ from Wolf to Brenner
• 06/07/2020 Demolition of the exploratory tunnel diaphragm in the municipality of Steinach am Brenner
• 24/11/2021 Thanks to the Serena TBM, after 3.5 years and approximately 14.2 km of excavation to the north, a BBT tunnel reaches the Brenner border for the first time: the exploratory tunnel, at a depth of 1,450 m
• 18/09/2025 With the completion of the exploratory tunnel to the south, a BBT tunnel connects Italy and Austria for the first time.

In May 1994, a railway bypass, the so-called ‘Inn Valley Tunnel’, was inaugurated south of Innsbruck. This 12.7 km long tunnel connects to the Brenner Base Tunnel. Passenger and freight trains travelling on this route will not only pass through the BBT but also through several kilometres of the Inn Valley Tunnel. This line, which is 64 km long in total, will be the longest underground railway connection in the world in the future.

The Brenner Base Tunnel will offer a viable alternative for freight transport and open up a new dimension of travel for passenger traffic.

Project progress

The Brenner Base Tunnel is a tunnel system with a total length of 230 km. To date, 204 km of tunnels have already been excavated:

•    100 km – train tunnels

•    57 km – exploratory tunnel

•    47 km – other tunnels

For further information on the project click here, at/16 and it/103 for the tunnelbuilder archive and also visit https://www.bbt-se.com and  https://www.webuildgroup.com/. 38/25.