Percy Jackson and the Olympians: Legends

The lowest pit of Tartarus and the universe. It was limited by an infinite stone ceiling, but there was a big hole on a part of it. Sunlight entered Tartarus through it, which indicated it lead to the surface and was an exit to the Underworld. 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.