Context: The construction of India’s first underwater tunnel in Kolkata under the Hooghly river for establishing metro connectivity between Howrah and Kolkata is underway and will be made fully functional by 2023. 
About India's first underwater Metro

• Of the 16.6-km-long east-west stretch, 520 meters will be under the river bed of Hooghly river. 
• The tunnel corridor is built 33 meters below the riverbed and will connect Kolkata to Howrah.
• The Kolkata East-West Metro line is being constructed by Kolkata Metro Rail Corporation (KMRCL) and has the underwater tunnel that will go beneath the Hooghly river. 
• Speaking on the facilities and safety measures being provided in the project, there will be walkways in the tunnels for evacuation of passengers in case of emergency.
• Almost 80 percent of the work of the East-west Howrah Metro station is completed and expected that the full-fledged service may start from 2023.
• Important Passage work has also been done keeping in mind that if a technical problem arises inside the water tunnel area, passengers can be taken out through the special passage.
• The station is being built under the Hooghly river at a depth of 33 meters. 
• 80 percent of work has been completed while 20 percent is yet to be finished.

Important points

• Tunnel dug 30 metre underground
• Total distance between impervious layer and river is 13 metres
• Total length of track is 16.6 km
• Automatic train with no driver. Can be driven remotely too
• Type of track is ballastless and the length of underground track is 10.8 km
• Type of gauge is standard gauge 1,435
• Will connect Howrah and Sealdah — the busiest railway terminals in West Bengal
• Number of tunnels is two
• Six air-conditioned coaches in each train
• Number of passengers per train will be around 1,000