Krishna's Butter Ball is one of the interesting site in Mamallapuram. It is very different from rest of the attractions carved out of stone here.
Very close to Arjuna's Penance on the hill slope is a huge boulder. This massive rock is perched on the slope of the hill opposing the all laws of physics. This has been standing in this position for centuries now without any support. Area around the boulder is so slippery that kids use it as a natural slide.
For some reason it is famous by the name "Krishna's Butter Ball". I guess it resembles the shape of a butter ball and Lord Krishna loves it. Beyond this I have no other clue. But this natural balance is worth all the attention.