Which is the only region in India below sea level?

"Kuttanadu in Kerala, India is the lowest region of India, with 500 square kilometres of the region below sea level. Its elevation ranges from 0.6 m above to 2.2 metres below sea level."

https://en.wikipedia.org/wiki/Kuttanad

The area is also called Thirst.

Despite being enriched by two monsoons and four rivers, Kerala's Kuttanad region suffers from acute drinking water shortages.

It is a waterlogged stretch of about 110,000 hectares; and 50,000 ha of the region are even 60-220 centimetres below sea level. For the better part of the year, most of the land is submerged in water. It has the distinction of being one of the few areas in the world where farming is carried out below sea level. Four major rivers in the state - Pampa, Meenachil, Achankovil and Manimala - are discharged into the region. For about 1.8 million people of Kuttanad, it is water, water everywhere.

And yet, when it comes to drinking water, they need to wait for the calls that herald water supply twice a week, often at night.