Mumbai Metro Rail Corporation Ltd has completed burrowing through about half the 33.5-km route of the Colaba-Bandra-Seepz Metro Rail, India’s first fully underground Metro system. It will also be India’s second under-river subway tunnel which will pass under Mithi river in Mumbai.

Why is this a milestone in tunnelling?
The pitfall here is not the depth. It is the challenges that come with tunnelling below a water-body. There is a possibility of mud and water rushing into a just-constructed tunnel section. The only other under-river tunnel for a mass transit project in India runs beneath the Hooghly in Kolkata, completed two years ago.

 Typical illustration of a Tunnel Boring Machine excavating through rock.

 Typical illustration of a Tunnel Boring Machine excavating through rock.