La Gias is the Earthling term for a mystical world that occupies the center of the Earth. Simply digging straight down to La Gias is an exercise in futility for most people; it is as much a mystical place as a physical one. However, there are a number of natural gates and paths, and some people know how to open unnatural ones.
To natives, 'La Gias' refers properly only to the area around the kingdoms of Langran and Shutedonia, and most of the others will instead claim to be from whatever their kingdom or town of birth is. Earthlings, however, generally class it all together, referring to the entire inner Earth as La Gias.
La Gias appears much like the surface of the Earth: there are mountains, seas, and vast expanses of any number of climes and conditions. The big difference is that the whole world is on the inside of a sphere, rather than the outside. There are numerous kingdoms in La Gias. Some of them are at war with one another; others simply exist, as idyllic, fantastic worlds, isolated from the world above. Not all of them are just as easily accessible from Earth, or from each other; some places don't realize anywhere else exists but their little patch of La Gias, while others have common contact with Earth. Much of La Gias is unmapped, and it is enormous - likely bigger than Earth itself.
Many of them, however, do not follow normal physical laws entirely. Quite a few them have magic in regular, if not common, use. Others are simply set up to look like outside rather than inside the sphere of the Earth. Nonhuman species exist, and some of them even have their own mechs, such as the Beastmen.
La Gias' kingdoms and countries have little in common other than a shared existance in La Gias. The general unit of trade, if they trade at all, is barter, and the kingdoms vary immensely from one another both in culture and, sometimes, in physical laws. There are places where gravity does not exist in La Gias, and other places with heavy elemental or spiritual influence.
The common Earthling does not know about La Gias and probably wouldn't believe you if you told them. However, all of the major factions have had contact with at least some people from the Inner World, even if they don't know all of the countries and cultures there, and as such most militaries at least mention the possibility of it existing to their elites.
La Gias is an expansive place where all settings that cannot really exist on a modern Earth are adapted to. Fantasy is the primary setting type, but others exist, such as the Gurren Lagann badlands and other, ancient empires. If it's not on Earth or in space, but is appable on the MUSH, it's probably in La Gias somewhere.