Muse Cavern

The Cavern Of The Muses or alternatively the Muse Cavern is one of the largest caves in the world, rivaled only by those that contain the rare Hell Islands, and the largest complex of caves above sea level of anywhere in the world.

Location
The Cave's entire area is contained inside the Red Line, and is only accessible through a single hole that connects to the upper parts of the Red Line, other man made entrances and exists exist and they link to the seas below.

The Exact location of any of these is unknown because the cave's length is roughly 40% of the red line, and seems to have exits that stretch among all 4 blues and even the grand line itself, allowing for passage to the other side through the cave system, this however means that one can't bring their ship along, unless the ship can mobilize through uneven ground and tight spaces.

The knowledge about the cave seems to be pretty obscure, very few people acknowledge it's existence for no given reason, and the world government doesn't seem to bother acknowledging it too much, at least publicly.

Ecosystem
The Cave is so massive it spans inside it it's own atmosphere and biomes, one can find completely deserted places and tropical forests in it, all growing out of the solid rock or even the sides. Some of the species that can be found living inside the Muse Cavern include:

Geological History
No one's rally certain how such a huge cave came to be, however it's theorized the cave is not natural and that something else might have taken part in the creation of it. It's age does leave the options of humans mute, so whatever did it was probably pretty big or pretty powerful rather than intelligent.

Trivia

 * The Cave is based on the Sơn Đoòng Cave the biggest cave in the world.