In March 2024, India inaugurated its first underwater metro tunnel — a 520-metre crossing beneath the Hooghly River that took 15 years, two giant boring machines, and decades of engineering nerve to build.
The Hooghly River does not give ground easily. For centuries it has separated Kolkata's commercial heart from the westward sprawl of Howrah — a crossing so significant that its 1943 cantilever bridge (the Rabindra Setu, still one of the world's busiest) became an emblem of the city itself. So when engineers first proposed running a metro line beneath the river in the early 2000s, the project carried a weight beyond its geotechnical complexity.
On 6 March 2024, Prime Minister Narendra Modi inaugurated the Howrah Maidan to Esplanade section of Kolkata's East-West Metro — and with it, India's first underwater metro tunnel. The 15 years between conception and ribbon-cutting were not smooth; the section was delayed repeatedly by land acquisition disputes, a devastating underground collapse in 2019 that cracked historic buildings above, and the disruption of the pandemic. What emerged at the other end of all that time was a genuine engineering landmark.
The 520-metre crossing
The underwater stretch is approximately 520 metres long, passing beneath the Hooghly at a depth of around 13 metres below the river bed — and roughly 33 metres below street level at its deepest point under Strand Road on the Kolkata side. Two Tunnel Boring Machines, nicknamed Prerna and Urvi, each 6.27 metres in diameter, bored their way from launch shafts near Kolkata's Esplanade through the saturated alluvial soils beneath the river to emerge on the Howrah bank.
The soil under the Hooghly is notoriously unstable — a mix of soft silty clay and running sand that behaves like wet concrete in dry conditions and like liquid under pressure. Engineers used compressed-air face support during excavation to prevent the tunnel face from collapsing. The machines also had to be steered to avoid the footings of the old Howrah Bridge and to stay within tolerance of a track alignment that had to arrive at the Howrah station box within centimetres.
The 2019 collapse — when a section of tunnel excavation caused a building on Durga Pituri Lane to sink and crack, killing one person and injuring several others — halted work for months and forced a complete review of grouting and monitoring protocols. Engineers added real-time settlement monitoring on every structure along the tunnel alignment and tightened face pressure controls before resuming.
Why this matters beyond Kolkata
India's cities are running out of surface and elevated real estate for metro rail. Mumbai, Delhi, Bengaluru — all are pushing lines deeper underground to avoid the tangle of utilities, heritage buildings, and traffic that crowds the surface. Mumbai's coastal lines will eventually face estuary crossings. Bengaluru Phase 3 includes tunnelled sections under dense urban areas.
The Kolkata East-West Metro crossing is the proof-of-concept for underwater metro construction in Indian soil conditions. The techniques refined under the Hooghly — pressurised TBM excavation, real-time settlement monitoring, deep waterproofing details — are now documented in KMRCL's project reports and available to every future Indian metro authority facing a similar river crossing.
The corridor today
The full East-West Metro corridor runs 16.6 kilometres from Howrah Maidan in the west to Salt Lake Sector V in the east, where Kolkata's IT parks are clustered. As of the March 2024 inauguration, the Howrah Maidan–Esplanade section (including the underwater stretch) opened, completing the connection under the Hooghly. The route now offers commuters a direct, air-conditioned ride from Howrah's rail hub to Esplanade without fighting the traffic on the bridge — a journey that once took 30–45 minutes by road on a bad day and now takes under 10 by metro.
Fifteen years. Two boring machines. One river that has been crossed by pedestrians, trams, ferries, and the iconic cantilever bridge. Now it has a metro line running beneath it — and India has its first underwater rail tunnel.