A natural stone bridge, interwoven with the Hindu mythology - the Ram Setu Bridge – also known as Adam's bridge is a long string of coral reef rocks that connects the Rameswaram Island in Tamil Nadu, India to Mannar Island, off the north-western coast of Sri Lanka. This bridge is believed to have been built across the sea for the Hindu God Sri Rama by Hanuman and his associates when they marched across a small strip of sea to Lanka to fight King Ravanna and bring back Seetha (Sri Rama's wife) who was in siege. In the Sanskrit epic Ramayana, it is mentioned that they jerrybuilt a passable stone bridge across the sea to cross over to the island country.
These big fragments of reef rocks were exposed after the worst devastating Tsunami that struck the eastern coast of India and other places on December26, 2004.These huge coral reef rocks actually float in the water and never get submerged because of lots of void spaces in them. It may be possible for humans to use these series of floating stones and cross a huge water body.
At Rameswaram I myself saw in some places huge stones weighing more than 30 kg floating in big water tanks without getting submerged. After observation first hand logical explanation is - invariably almost all the floating stones are coral reef rocks made of calcium carbonate. They have lots of very small hollow spaces once occupied by colonies of tiny animals that normally build the reef in the coastal marine waters; most reefs were built by stony corals eons ago.
The numerous small air cavities in the rocks are not even and are found to occur unevenly inside them and also on the surface. Further, the weight of the water displaced by them is more than their own weight. As for buoyancy, the following inference can be made: Almost all floating stones at Rameswaram are coral reef rocks with variable density and air cavities.
The floating rock segment exposed to the viewer above the surface has more air cavities and less density. On the other hand, the bottom segment (immersed in water) has more density and less air cavities. Further, it is likely, it may be due to salinity of the floating medium - water.
No matter how you drop the stone into the water, it automatically realigns itself in such a way that less denser portion with more air cavities is turned towards the viewer.