India's first underwater metro train project was launched in:

India's first underwater metro section was opened in Kolkata, West Bengal, on the East-West Metro (Green Line). Prime Minister Narendra Modi inaugurated it on 6 March 2024, and passenger services began on 15 March 2024.

About the underwater metro:

• It runs through twin tunnels built under the Hooghly river, connecting Howrah (on the west bank) with Kolkata (on the east bank).
• The underwater section between the Howrah Maidan and Esplanade stations is about 4.8 km long, of which about 520 metres runs directly beneath the riverbed.
• The tunnels lie about 13 metres below the riverbed and about 32-33 metres below ground level.
• A train takes about 45 seconds to cross the river section.
• Howrah Maidan to Esplanade passes through Howrah station, which, at about 33 metres below the surface, is the deepest metro station in India.
• The tunnels were dug using tunnel boring machines (TBMs) and are lined with special concrete segments with sealing gaskets to keep the water out.
• The East-West Metro also connects to Sealdah and Salt Lake (Sector V), linking the two busiest railway stations of the city, Howrah and Sealdah.

Kolkata Metro's other "firsts":

• It was India's first metro system, starting on 24 October 1984 between Esplanade and Bhowanipur.
• It became the 17th zone of Indian Railways in 2010, the only metro run directly by the railways.

Other water-based urban transport in India:

• Kochi Water Metro (2023): India's first water metro, using electric hybrid boats to connect Kochi with nearby islands.
• Mumbai's Coastal Road and Mumbai Trans Harbour Link (Atal Setu, 2024), India's longest sea bridge, about 21.8 km long.

Why not the other options?

• ✗Mumbai has the Atal Setu sea bridge but not the first underwater metro.
• ✗Chennai has an underground metro, but not under a river.
• ✗Kochi has India's first water metro (boats), not an underwater metro.