the underworld

The Underworld is a term used to describe the subterranean of Panthea, and more specifically, the endless maze of caves that permeates Panthea's rocky crust. Panthea has such an incredible amount of caves that almost every part of it has a tunnel running throughout it, and these tunnels extend possibly thousands of kilometers down, although their true depth is unknown. These tunnels are deep and numerous, and also vast in scale. Some tunnels can be as wide as valleys, with their rooves the height of mountains, and they can lead to even more massive hollow chambers of earth that can house entire ecosystems and weather cycles. And these caves aren't simple dark, cold, rocky tunnels. They're often bright, illuminated by glowing crystals or bioluminescant animals. They aren't always cold either, some regions underground can reach temperatures hot enough to melt stone into magma, which results in caves housing rivers and lakes of magma, and is also the source of volcanoes up at the surface.
The diversity of the caves beneath Panthea is truly staggering. There are a great number of variables that can affect what a cave is like. For one, size is a factor. Some caves can be so small that someone could barely fit their arm in, while others are such vast open areas underground that single chambers can be as large as entire islands. Another large factor is what type of stone the cave is formed from. Some caves have smooth walls, some are made up of huge bouldery masses, but many are full of stalagtites and stalagmites, while some have none at all.
Another variable is temperature. The temperature underground varies greatly by region. Some parts of the underground can be so hot that rock melts into magma, and some other places are so cold the underground rivers freeze solid in them. Between these however are more moderatley heated caves, that usually have temperatures ranging from something similar to a boreal forest, to the temperature of a hot jungle.
Another factor is how illuminated they are. Some caves are absolutely pitch black, while others are illuminated by fire and lava inside them, while others are illuminated by the natural miracle that is sun stone, a crystal that gives off life giving light and warmth that allowed plants to grow deep underground, far away from actual sunlight. Sunstone can form in cold and moderate temperature caves, but they cannot form in extremely hot caves, such as the ones that contain lava.
Another major factor is how much water is present in the cave. Some caves are full of flowing streams, or great rivers, or even waterfalls. Water can sometimes even completely fill a cave, while other times it will freeze and create icey flat surfaces on cave floors. In rare instances water will flow into a lava cave, creating great plumes of steam too. Water is also essencial for life to thrive, and usually, the caves with the rivers flowing through them have the most life. Water also can have the unique effect of creating weather patters on some caves. Because some caves are extremely massive, and often have lakes within them, this means clouds are able to form and rain can fall. There are also some hot caves that completely lack water, and in some instances ever desert caves can form.

Life In Caves

Another factor and one that is affected by all other factors is life. Because caves can have sunstone, which provides sunlight, and water can flow through caves, and rain can fall in especiallly large caves, plants can easily grow inside of caves. Animals will then evolve to eat these plants, then other animals will evolve to eat those animals, and then there's an entire ecosystem. These ecosystems are extremely diverse in the Underworld because although almost all caves are connected in some way, life has spread to just about every part of the Underworld, although various ecosystems will remain separate from eachother, meaning many idividual sections of cave will have their own unique species. This extreme biodiversity is also greatly affected by all of the previously described factors that affect how caves are formed, and the tunnels and chambers that run deep underneath Panthea are some of the most diverse in the world.

Inhabitants

Intelligent species

The underworld is home to a rich diversity of life and cultures. Most notably is the Subterranean race, which has inhabited the underworld for hundreds of thousands of years, creating a complex series of city states, tribes, clans, and empire deep below the earth. These subterraneans are related to humans, but they are taller, possessing white skin, large eyes, thicker bones, and larger muscles.

Flora and Fauna

almost all plant and animals in the underworld are unique to it. The flora of the underworld are giant glowing mushrooms, ferns, cave grasses, vines that hang from the cave ceiling, wall vines, bushes, and even some trees.
The fauna of the underworld consist of bats, giant spiders, giant centipedes, giants snakes, giant bats, cave fish, cave birds, lizards, many species of bugs, cave octopuses, giant lizards, salamanders, and Salanjur.
Deep underworld fauna consist of several different species of creatures that all possess no eyes and transparent or white skin, due to the extreme darkness.

Type

Underground / Subterranean