Drilling of the seven-km undersea route of the Mumbai-Ahmedabad rail corridor is underway to ascertain soil condition of India's first bullet train path.

Passengers will get the thrill of riding under the sea, a first in the country, near Thane at a maximum speed of 350 km per hour in the upcoming high speed train project connecting two major metropolis.

"Soil and rocks below the 70-metre-deep see are being tested as part of the geo-technical and geo-physical investigation undertaken for the entire project," said a senior Railway Ministry official, adding "the test will also cover the 21-km-long underground tunnel between Thane and Virar."

Barring the 21-km-long tunnel, most part of the 508-km- long corridor is proposed to be on the elevated track while there will be a stretch after Thane creek towards Virar which will go under the sea as per the detailed project report by JICA, the funding agency of the project.

Railways opted for an elevated corridor to avoid land acquisitions and the need to build underpasses.

The tunnel was necessitated to protect the thick vegetation in that area, said the official.

The geo-investigation is crucial for the project as it would ascertain the bearing capacity of the soil below 70 feet at the sea.

Estimated to cost about Rs.97,636 crore, 81 per cent of the funding for the project will come by way of a loan from Japan. The project cost includes possible cost escalation, interest during construction and import duties.

The survey is likely to be followed by the final location survey to mark the alignment and exact spots for the pillars on which trains will run at higher speed to reduce the travelling time between Mumbai and Ahmedabad drastically.

Currently it takes about seven hours to travel between the two cities and the bullet train aims to reduce it to about two hours.

Construction of the corridor is expected to start in 2018 and is estimated to be completed by 2023.

JICA agreed to fund 81 per cent of the total project cost through a 50-year loan at an interest rate of 0.1 per cent and a moratorium on repayments up to 15 years.