In fact, this Lingam which had been worshipped by Lord Rama thousands of years ago, is now known as Ramanathar Lingam, and on Hanumanji returning and finding out that his Lord Rama had not used the Lingam that he had brought, he was quite disappointed.. But when Lord Rama came to know of this , he immediately pacified Hanumanji and gave the name of Lingam Kasi Viswanathar to the lingam brought by Hanumanji. And to this day, even thousands of years later, devotees are called upon to first do worship of Kasi Viswanathar before worshipping Ramanathar.
If you observe the temple, you will also find that the shape of this island temple resembles the Panchajanya (Vishnu's conch) and like the Tamil letter 'OM'.
Interesting part about this temple complex is that it has a total of 51 teerthams, of which 22 are located within the temple itself, and these large number of teerthams has made Rameshwaram very famous. You may only consider your pilgrimage to be complete only once you have taken a bath in the Agni teertham, where the waters are also considered to have medicinal qualities.