The Shiva Lingam here is a swayambhu lingam. The miracle is that the colour of the deity changes 5 times a day.
It is said that during Shiva-Parvathi wedding in Mount Kailash, the place caved due to the presence of all the gods. To balance the level of the earth, Lord Shiva sent Sage Agasthya to the South.
He was disappointed as he could not witness the marriage. The sage installed a lingam behind the main lingam in the temple to worship Shiva, and pleased with his worship, the Lord with his wife, appeared in front of the sage and married again here at this holy place.
The unique thing about this temple is you can see all three deities Brahma, Vishnu and Shiva here.