The boiling rock is a prison on rock in the middle of a boiling lake on a volcanic island. It is well known because it is "impossible" to escape. Obviously it was possible, but for the roleplay it is impossible to escape, but not to be rescued from. Sokka and Zuko travel here to find Sokka's father, Hakoda, and free him. He isn't there, yet, but Suki is and when trying to escape trouble ensues.