Thonsepar, aka St. Mary's Islands, aka Coconut Island, sits in the Arabian Sea just off the coast of Karnataka state of India. The island is known for its distinctive geological formations, specifically the columnar joints (hexagon-shaped interlocking columns) that protrude from the earth.

The columnar joints vary in height, but the tallest of them measures approximately six meters (20 feet). According to geologists, the columns formed by subvolcanic activity roughly 60 million years ago as Madagascar split off from India.

The shape of these joints is the product of a physical process that can happen as melted lava rock solidifies. As a large amount of molten material cools, it starts to contract, creating cracks in the newly solid surface. The cracks extend down as the rock below the surface continues to cool, resulting in these unique rock formations. You can see similar formations at the Giant's Causeway in Northern Ireland.

Thonsepar was classified as a National Geological Monument in 2001 by the Geological Survey of India.