Caverns

Beneath the rift valley are a series of hydrothermal caves, embedded mostly in the carbonate rocks of the Central plate craton, deep beneath the surface and the many layers of less penatrable rock above it. These caves were, in history, open to the surface, and during that time were populated with an abundance of creatures. Over evolutionary time, the inhabitants of the caves moved further and further inward, colonizing their own unique ecosystem in the caverns and tunnels, nourished by the rivers that ran through them and the chemosynthetic producers at the hydrothermal vents in the deepest parts of them. When the caves' mouths closed to the surface, this ecosystem continued to thrive as it was forgotten by those above it.

In the years leading up to the cataclysm, the increased seismic activity disturbed the area and a number of earthquakes resulted in the opening of these caves to the surface once more. The entrances were few and far between, but they were found- both by tellurans curious of the depths, and by nightlings, curious of what lay above them. This marked the first contact between nightlings and sunlings in thousands of years.

The interior of the caverns is a complex, winding labrynth characterized by broad, tall caverns of immense size connected to each other by long, narrow tunnels. Much of the wall, floor, and ceiling, especially in the deepest and shallowest caves, is covered by various species of fungi, many of which exhibit bioluminescence as a method of attracting insects which spread spores in much the same way bees spread pollen. The deepest caves have an abundance of life due to their close connection to the chemosynthetic production taking place in the sumps of the caverns; the shallowest caves have an abundance of life due to their connection to the surface and the trickle of nutrients that flows downward into the caverns. It's the intermediate depths that fall into dark and nontenancy.

Aquatic Life

The primary producers of this ecosystem are chemosynthetic organisms that originated around the numerous hydrothermal vents at the very bottom of the system, but have since spread to the majority of the cavern space. The sump ecosystem looks much like what deep-sea vent systems do, but because of the isolation of the caverns, there are some notable differences.

There are significantly more mobile species that swarm the vents in the caverns than in the oceans- animals in the sea have no reason to stray far from the vents as there is nothing but empty water for miles, however, in the caverns, there are walls and even the surface of the pools to attract animals for different reasons. Species of shrimp find food at the vents, but cling onto the cavern walls to lay their eggs. Eels feed on these shrimp, as well as crabs, then nest in cracks and holes in the cavern walls. Fish travel back and forth between the cooler surface where their delicate eggs can hatch, and the boiling vent area where they eat the microorganisms that swarm there. The back-and-forth migratory nature of many of these species allows for the food chain to make the jump to the surface, where it can then expand throughout the caverns.

Production is not just contained to the springs, though: water flows throughout the caves, allowing life to flourish in the whole of the caverns' area. It's the most concentrated at the springs- tube worms and bivalves and shrimps are among some examples of organisms with endosymbionts that chemosynthesize the hydrogen sulfide in the water as a food source. But there is also a unique clade of coral-like organisms that grow in the waterways. They have endosymbionts like marine coral, but the difference is that theirs are not algae, but chemosynthetic bacteria. Therefore, the colonies are primarily are filter-feeders, and as such feature different morphologies than surface corals that compete for light: cavern corals' main competitive resource is space.

Space is freed by a species much like marine bone worms that feed on the calcium carbonate skeletons that corals leave when they die: this prevents overcrowding of the limited space comparative to a marine reef. Further, with no wave action to buffet the organisms, they grow significantly more delicate than their marine counterparts. Some have thin, tendril-like or twig-like branches in order to optimize surface area for catching nutrients, while many are tube-shaped or otherwise sponge or hydra-like.

Terrestrial Life

There are no mammals native to this system- a cavern is a hostile environment for a mammal to live in full-time. Most of the caverns' creatures are reptiles and amphibians, but there are also some birds, mostly small insectivores, some fungivores and carnivores. The line between bird and reptile can be a blurry one, but cavern bird species especially tend to be more reptilian than their surface counterparts, and usually resemble feathered dinosaurs more than modern birds. All of the caverns' birds and reptiles are speciated from surface counterparts: very very few macroorganisms are completely original to the caverns, and no fully terrestrial animals are.

Most bird species found in the caverns are capable of echolocation to aid in their movement throughout the caves: many small birds whistle, while nightlings also click, though this ability is no longer true echolocation and has instead been incorporated into their language.

For reasons not fully understood, many of the caverns' surviving terrestrial species are hexapods: hypotheses have included a use for extra limbs in scaling walls, selective pressure within the caverns, or a random bottleneck event that left only hexapods inside the caves.

Many small amphibians developed bioluminescence to attract insects that they feed on, and some even mimic fungi in other ways as well, like the carnival-backed salamander, which resembles a species of slime mold that beetles eat, or the puffball frog, whose body can be positioned to mimic a mushroom that attracts significant amounts of flying insects.

Plant Life

There are no true plants living in the caverns, there is no sunlight for them to photosynthesize with, however, there are a vast variety of fungi in their place. Many of the fungi that inhabit the caverns are bioluminescent- their light attracts insects that help them spread their spores. The open nature of the caverns in early evolutionary development was integral to the emergence of the light system- insects from aboveground (equipped with vision, which troglobitic insects did not have) interacted with cavern species of fungi often enough for the development of bioluminescence to be a benefit. By the time the caverns closed off to the majority of trogloxenes, there was an established ecosystem of luminescent fungi and their 'pollinators.'

Seasonality

Being so closed-off from the surface, the caverns do not exhibit any of the typical seasons terrestrial life is used to. They are, however, connected to the seasons of the surface in one way: water. Water from the surface leeches into the caves and it can do so either very slowly or very fast. In the spring rainy season, excessive water infiltration results in flooding of the cavern rivers- swelling the currents and increasing nutrient density as they are carried from the surface. For this reason, caverns maintain an annual connection to the surface even when they don't know the cause.