Percy Jackson and the Olympians: Legends

The Realm Below is the lowest plane of the universe. It was limited by an infinite stone ceiling, but there was a big hole on a part of it. Sunlight entered through it, which indicated it lead to the surface and was an exit to Tartarus. It was unknown why it existed or if it was made by someone, but it seemed to be large enough for a God or even a "small" Titan, like Typhon, to go through and seemed to be very old.