Two top-order tunnel boring machines, nicknamed Rachna and Prerna, are digging India's first ever under-the-river Metro tunnels in Kolkata at full speed. If all goes through, then one could take a Metro ride under the Hooghly river in December 2019. Tunneling for the project began at the Howrah end in April under the Hooghly river. The tunnels are 520 metres long and about 30 metres deep. The under-the-river Metro ride will be all of 60 seconds long.