India's first underwater train tunnel will be built by Indian company Afcons Infrastructure and Russian firm Transtonnelstroy as part of the Rs4,678 crore ($1.03bn) metro project in Kolkata.
The companies will build the rail route from central Calcutta to Howrah Maidan, running under the Hooghly river bed, involving construction of a 3.89km stretch including 520m underwater.
The tunnel under the Hooghly will be nearly 30m under the water surface and 15m below the river bed.
Work, which includes building three underground stations, is expected to finish in 42 months.
The Rs938 crore ($206m) contract for the underground route has been awarded by the Kolkata Metro Railway Corporation.
Dubai-based firm Atkins will provide consultancy services for the tunnel work, which includes monitoring and studying the impact of tunnelling on the buildings and structure above ground.