Subterra is the collective term for all cavities beneath Threa's surface. As such Subterra is huge and technically the largest region of them all. It's the world beneath the world. Most people only see its upper layers, the caves and tunnels close to the surface. A shame, considering how gorgeous Subterra's depths are. They may not be as colorful and lively as the Starless Jungle, but curious rock formations and glowing crystals and mosses have their own charm. Some caverns are impressive sheerly because of their tremendous size.
No nation lies claim to Subterra's depths; they do not offer enough to sustain a population. Food is hard to come by. Fresh air is scarce in some places. At least the caves nearby the surface are hospitable and house plenty of peoples. It's common for people who live in the mountain ranges to seek shelter in caves or even use them as proper homes after improving them with furniture and by applying masonry skills. Especially the Hozlin clans use Subterra as a home.
Subterra consists of multiple colossal caverns and long, winded tunnels of various sizes, ranging from too narrow for a mouse to wide enough for a Hozkul. Most underground ways are close to the surface, but some go far down and reach lava filled caves deep in Threa's crust. Not all tunnels are natural either; eager miners, especially those of the Hozlin clans, work day and night on forming the flesh of the underground. Additionally, occasional tectonic activity changes the underground world up. It's hard to properly define the shape of to Subterra due to that. It's important to note that Subterra is not one large connected tunnel network. Subterra consists of many individual, isolated cave systems and tunnel networks.
Fresh water flows into Subterra through various connections to the surface. The water naturally erodes a path through the rock, creating firmly bedded subterranean rivers that feed underground lakes. Many of the deep underground lakes have hydrothermal vents.
It is very important to note that Subterra does not only consist of the deep, dark underground. The bottom of many rifts, canyons, gorges, and sinkholes are so deep that they're considered part of Subterra, or at least an important aspect of it, despite the fact that sunlight reaches them for a few hours each day.
Most tunnels and caves can be found in and beneath the Aezrikka Zule. Many of them are artificial in nature; Hozlin slaves, driven on by their Aezkul taskmasters, dug labyrinthine fortresses deep into the mountains. Subterra becomes chaotic beneath the Starless Jungle. Plenty of entrances can be found here, such as sinkholes and rifts. Most of the tunnels and caves there are flooded due to the copious rainfall on the surface. Overall, the underground region beneath the Starless Jungle is amazing habitat for aquatic and amphibian life forms that are adapted to darkness, as it combines a copious amount of water with high temperatures and sparse sun light. Some of these creatures make the underground beneath the rainforest extremely dangerous. The regions further north, so the Great Savannah, the Forest Lands and the southern Boreal Lands, aren't as habitable, but still good. Water is plentiful and there are many large caves, especially in and under the mountains. The Forest Lands, being the most mountainous out of those three regions, has the most caves. While there surely are tunnels and caves beneath the Frozen Wastes, they are concealed underneath a thick layer of ice and snow and most likely not noteworthy. It's far too cold in that place as that anything could live or settle there. Whatever subterranean lands may be located under the ice, they will be a mystery for a while and of no interest to the inhabitants of the underground, except for the few small organisms that can endure the hostile environment.
The climate one experiences in Subterra depends on the depth of one's location. Humidity and temperature of the shallow subterranean layer are very similar to that of the surface. The deeper the cave, the less it is influenced by surface and its seasons. Rock is a good heat storage. While the winter cools the surface, the rocks, still warm from summer, heat the cave. In summer the sun heats up the surface, and the cold rock absorbs any excess warmth in the cave. Most caves hence have a cool but comfortable 10 - 12°C (50 - 53°F) If one goes deeper than 50 m (~196 ft) beneath the sea level, then they'll notice the air and rock becoming warmer. They're experiencing the heat given off by Threa's hot, molten core. It is roughly 40°C at a depth of 800 m (2600 ft)! Naturally, weather can't be observed once one gets far enough away from the surface.
The ecological system of Subterra is complex. It's separated into four parts, which from highest to lowest are: the surface zone, the twilight zone, the dark zone, and the deep zone. The surface zone are the cave entrances, canyons, rifts, sinkholes, and similar that connect the subterranean layers to the surface. Sun reaches the surface zone for a few hours each day, so as long as water is within reach some hardy plants. such as local brushes, grasses, and smaller trees. can grow in it. The twilight zone sees far less light. Here only the toughest of plants, for example mushrooms, lichen, and mosses, can survive. No plant can make it in the dark zone or the depths.
Animals of the surface zone tend to simply be local animals that find food on the surface but seek shelter in subterranean cavities close to the surface. Creatures of the twilight zone are adapted to darkness and feed on the various mosses and mushrooms. They're usually small in size, unless they're predators that prey on the surface creatures that enter their caves. Only carnivores can survive in the dark zone. There are no plants, so herbivores can't survive here. The carnivores usually eat surface and twilight zone creatures. Some predators even leave their underground homes to hunt on the surface at night.
An alternative form of sustenance, at least for the aquatic life forms, are the various tiny critters of the deep zone. Said critters consume the minerals that hydrothermal vents blow into the surrounding water. Said vents are located within some of the lakes of the deep zone. Naturally the little organisms are adapted to the extremely hot temperatures of the water surrounding these vents. Underground fish and similar, which are also adapted to the temperatures, swim down to the vents and eat some of the critters. Because they use the twilight and dark zone to reproduce and take care of other matters, they sometimes end up as prey to a bigger, land base subterranean predator. Such is the food chain in some places of Subterra.
A good example of beasts that thrive in the twilight and dark zone are cavern crawlers. They can be found beneath the Starless Jungle, the Aezrikka Zule, and more rarely the Great Savannah. They resemble gigantic insects and live in large hives. The crawlers have managed to domesticate fire maggots, which share a habitat with them. The crawlers feed the maggots with whatever organic matter they can find, so that they can in turn drink the fatty liquid the maggots produce. A lot of the organic matter comes from the surface, but a considerable amount also comes from other creatures that live underground. Cavern crawlers are dangerous predators. Other noteworthy creatures are the fearsome blindseekers, which live beneath the Forest Lands and hunt in the deep, dark and twilight zone. They're blind, but are extremely sensitive to vibrations. When the blindseekers feel miners digging away at the rock nearby, they will hunt the workers down and try to consume them with their fang-filled maws. Blindseekers are quite large and can even threaten a kul's life, if not a Hozkul's life. Hozkul are one of Subterra's mightiest inhabitants, despite only using it as shelter. These huge, heavy kul can successfully fight most of Subterra's beast, except for hordes of cavern crawlers. Hozkul only inhabit the valleys and caves of the Aezrikka Zule.
This plant is a short, leafy brush that primarily grows in the twilight zone of caves in the Forest Lands. It doesn't look like much, even when its blue flower is blooming. A layman won't consider this plant valuable or useful. It does not look special or unusual after all, but it's actually very rare. The conditions that have to be met for the truth plant to grow are very specific and not easily met. Its roots, which have a light brown skin and faintly yellow flesh, are the main ingredient for truth serum. To create said serum, the roots have to be mixed with other ingredients and properly cooked. When a person ingests the truth serum, they cannot knowingly speak a lie. They aren't even aware that they have been poisoned. If they are made aware, then they can refuse to speak at all, but still cannot lie. A truth serum is quite a handy tool for judges, spies, inquisitors, and other seekers of the truth.
Hunting in Subterra depends on the region. It can generally be said that the deeper one goes, the less successful hunting trips are likely to be. There simply aren't any animals that are worth hunting, except for Subterra's dangerous predators. They're large enough to offer enough meat to be worth it, but also large enough to be a threat to the hunter. What makes hunting them even more rewarding is their very tough skin. Blindseeker leather is among the toughest one can acquire. Cavern crawlers, while being very dangerous, are also valuable prey. It takes a lot of effort to make their meat edible, because of their acidic blood, but their carapaces are acid resistant! They can be used to make acid resistant armor which in turn makes fighting cavern crawlers and handling acids easier. If one's just after food and doesn't care for hides and shells, then fishing is the best option. It's safer than hunting and even underground will feed a few people. Subterranean fishes aren't dangerous, neither are the various crabs and other water creatures found in Subterra. All one needs is a net or a spear and perhaps a source of light.
Agriculture is impossible in most of Subterra's zones. The surface zone can be used to cultivate plants that require little sunlight and don't require high quality soil. Some hardy potato species can survive these conditions, for example. In the twilight zone farmers can at most grow mosses and mushrooms. Nothing else can deal with the almost utter lack of sunlight. What grows in a given cave depends on the region of the surface.
While wood is naturally scarce in Subterra - it can at most be found in the surface zone - stone usable for construction is readily available. One just has to grab a pickaxe and knock them out of Subterra's rocky flesh. Any kind of stone can be found in Subterra, from granite over limestone to marble.
All kinds of ores are available in Subterra as well, but they are not equally distributed across its expanse. Iron, copper, lead, tin, silver, gold, and gems, Subterra has it all, somewhere. The hard part is finding the deposits, but once one is found the mining can begin and it will usually last a long time. Subterra's veins are rich. Coal and crude oil deposits can be found as well, but only near the surface.
Subterra is dark and scary. It's rather wet in many places, too. Yet it has its beautiful sides. It has caves of immense size, with and without stalagmites, rock formations decorated with minerals, and breathtaking underground lakes. There's few sights more gorgeous than an underground lake with glowing crystals at its bottom illuminating what would otherwise be a dark abyss. Some of the caverns in the depths are impressively vast, so vast that a torch could not hope to illuminate their ceilings. Cavern crawler hives are an impressive sight as well, but it's very dangerous to explore them. They look tidy, organized, and are always busy as long as its hive is alive.
The aforementioned crystals are an unique aspect of Subterra. They come in two variants - red and blue - and appear to permanently emit soft light. It's enough to grand sight to creatures with sensitive eyes, such as the Hozlin and cavern crawlers. Even above ground they retrain their soft glow, but it is so little it can only truly be seen when it is dark. However, the crystals can only be found deeper than 30 m (~98 ft) beneath the surface.
The Maws are a cluster of large chasms in the ground, which are in the western half of the Starless Jungle, near the Aezrikka Zule. They are between 10 to 1200 m (33 - 3950 ft) long and up to 5 to 200 m (16 - 660 ft) wide. Most of them are so deep that they reach into Threa's underground caverns. The deepest ones reach roughly 3 km (1.9 mi) in depth. All the rifts are parallel to each other, going from west to east. Their walls have a rocky, jagged surface, and some feature natural caves and tunnels. It's possible to climb the cliffs, but only advised to do so for experienced climbers. At the bottom of a Maw one either finds an ecosystem adapted to little sunlight or a lake. There's a few rivers that end in some of the Maws, filling them with water, feeding various subterranean rivers and lakes.
Local flora and fauna have completely claimed the Maws. Vines and similar plants grow along the walls and many animals, especially birds, use the tunnels and rock outcroppings as nesting grounds and homes. The lakes at the bottoms of some Maws are teeming with fish.
Multiple Riz tribes live around the Maws, but not because of the gold. They see the Maws as sacred places where one may sacrifice various things to appease the jungle spirits, or whatever else they may believe in. With religious vigor, the local Rizlin throw gifts to their deities down the Maws, 'feeding' them to make sure they do not hunger for the flesh of Rizlin. They believe the spirits are especially happy if they are fed with the flesh of lin and kul, as long as they are not Rizkul, so they're rather hostile to intruders. Naturally all the Rizlin are doing is feeding the fishes and underground beasts that live at the bottom of the holes.
One of the Maws' larger rifts has walls that are lined with numerous, rich gold veins, which reflect the sunlight when it shines down the hole at the right angle. The glimmering reflections are a rather pretty sight. Accessing the gold is difficult due to their location and the various critters that have made the Maws their home.