Rameswaram is where Lord Rama worshipped Lord Shiva before he set out to defeat the powerful Ravana and reclaim his kidnapped wife Sita.
In commemoration of this mythological event, a huge Ramanatha Swamy Temple exists, which is an architectural wonder. A corridor runs around the temple that is said to house a thousand pillars. In addition, there is a custom of undergoing ritual baths from a dozen odd wells inside the temple, that have the effect of purifying a person of all sins and vices. The first of those baths called the Agni Theertham (bath of fire?) is undergone in the shallow sea.
This is an important pilgrimage centre for Hindus the world over, and is as significant as a visit to Varanasi, the holiest place for Hindus.