Subterra is a collective term used to refer to the regions of Erudite where conditions are right for deep ekati to naturally form. Subterra stretches from x kilometres below sea level to the very centre of the world. It is loosely divided into three zones - the Shallow Mantle, the Deep Mantle and the Core. Subterra has a wide variety of natural features, including wind caves, which help drive processes such as tectonic movement and internal transfer of heat and matter.
Subterra is home to a wide variety of species and peoples, including some of the last known ekati-based life-forms in Erudite (it is also the only place where ekati-based life is known to form). Various civilizations exist here, and an international economy has formed thanks to a network of trade routes making use of the wind caves and other underground series of caverns.
The Shallow Mantle is estimated to be between x and y kilometres below sea level. Here, while deep ekati is capable of forming, it is still very rare. Deep ekati crystals can take centuries to form, and many deposits here have been depleted by mining. As a result, Dark Elves frequently have to trade with residents of the Deep Mantle for access to deep ekati.
The Shallow Mantle is the highest place reached by the wind caves. It has sizable cave systems, as due to the young age of Erudite, holes in the world haven't yet been closed by gravity and pressure. There are caves here large enough to hold entire cities.
The Deep Mantle is commonly considered to be between y and z kilometres deep. It is characterized by deep ekati being far more common. The wind caves run mostly through the Deep Mantle, where they transport heat and gases through the interior of Erudite. The Deep Mantle is where most ekati-based life forms are found, although some enterprising Dark Elves have braved these depths as well.
The Deep Mantle has less cave systems than the Shallow Mantle due to the greater pressure exerted on it closing most open spaces. However, the caves that do exist here are enormous, thanks to ekati and deep ekati deposits levitating rocks and allowing massive spaces to exist. The largest of the caves that make up Kohatu-Nui is thought to be large enough to hold the island of Kharnstheim.
The Core reaches from z kilometres below sea level to the very centre of the world, at w kilometres down. It is differentiated from the other layers by being almost entirely dominated by a single life-form, the ekati-based eldritch Discord. Discord makes up the bulk of the Core, and due to its ravenous appetite for organic life, none can be found at these depths. Only ekati-based life exists within range of Discord.
Due to Discord's composition, the Core is made up almost entirely of deep ekati, but other substances can be found at these depths, including metals like iron. Unusual formations, such as linear paths and complex geometric fractals, exist here, created by Discord itself out of sheer boredom. The Wind Caves don't reach all the way to the Core, but it is believed that if Discord were able to reach them, it would be able to quickly spread throughout Subterra and even reach the surface.