India’s first underwater metro service to be inaugurated on March 6: Key points

This will be India's first underwater metro tunnel

Prime Minister Narendra Modi will inaugurate India's first underwater metro tunnel during his visit to West Bengal's Kolkata on March 6. Here are some of the key things to know about the historic project, which is known as an "engineering marvel":

* The under-water tunnel is part of the Kolkata Metro extension project, which will connect Salt Lake to Howrah via Kolkata with a stretch below the Hooghly river. The underwater metro is part of the East-West Metro corridor.

* The bottom part of the newly-built tunnel is 26 metres from the river’s surface. The trains will ply 16 metres below the riverbed.

* It will be India’s first ever transportation project where a metro train will ply under a river.

* According to a report in The Times of India, construction major Afcons and Russian company Transtonnelstroy jointly built the tunnel. Afcons dug the Hooghly tunnels in April 2017 and completed them in July 2017.

* According to the ministry of railways, nearly 10.8 kilometres of the stretch will be underground while 5.75 kilometres will be on elevated viaduct.

* The ministry said that the project will be immensely helpful in easing the traffic congestion of Kolkata.

* Last year, Kolkata Metro carried out a historic test run of two six-coach metro trains under Hooghly river's underwater tunnel.

* The project, which was earlier slated to be completed by December 2021, also faced some delays due to accidents at Bowbazar in central Kolkata during tunneling work.