Two Indian companies have entered bids to build India’s first undersea tunnel for the high-speed rail line between Mumbai and Ahmedabad, Indian Express reports.

Larsen & Toubro and Afcons Infrastructure responded to India’s National High-Speed Rail Corporation’s (NHSRCL) technical tender for the 21km excavation on 9 February.

The project involves building a single-tube tunnel wide enough for two tracks.

The tunnel will run between Bandra Kurla Complex on the Mumbai seafront to Kalyan Shilphata to the east of the city.

A 7km stretch will pass under Thane Creek, an inlet of the Arabian Sea that separates Mumbai and the planned city of Navi Mumbai.

The unusually wide diameter of the tunnel will require a tunnel boring machine (TBM) with a cutter head of 13.1m, twice as large as those commonly used.

According to the NHSRCL, three TBMs will be used to make about 16 km of the tunnel, and the remaining 5km will use the new Austrian tunnelling method.

This tunnel will reach a depth of 114m below Parsik hill, near Shilphata.

With 12 stations, the Mumbai–Ahmedabad line will run for 508km at a maximum speed of 350km/h. It is being part-financed by Japan with a concessional loan of $10.6bn. It is expected to be completed by 2026.