Subterranean caves are speckled all over the craggy, porous terrain of the sea, but they are largely superficial, and occupy an insignificant portion of the crust or a mountainous formation. Some lead to very picturesque grottos glowing with bioluminescent organisms. Others are heated and frothy from the thermal fissure of a neighbouring undersea volcano. The marine life found within is mostly identifiable by scholars or wilderness experts, and typical to the area.
Over the past two millennia, however, a small handful of cave entrances have been discovered in Rebma, which do not share the same characteristics of those above-mentioned. The tunnel opening descends five hundred feet or more into chilly blackness; at times widening into huge, cavernous crossroads; other times branching out into an assortment of disorienting catacombs; or else wandering off on a seemingly endless tangent.
One such entrance is confirmed to be located on Morfilod's territorial holding on the southern end of the realm, one somewhere beneath the Palace, and one on the other side of the mountain under the Temple of Lir. There are also additional suspected, as yet unconfirmed entrances near House Ygrayne's holding to the west, and Bauquemare's to the east. These disparate locations have led to the popular belief that the cave systems are so vast as to be connected to one another, far beneath the seabed crust, and the entire thing is referred to as The Great Labyrinth. Every so often, a bold individual or team has undertaken to explore and map out its many tunnels; the deeper they went, however, the greater the chance of their becoming trapped, stranded, or hopelessly lost. Beyond a certain point from the known entrances, there has apparently been no return.
Within what little has been successfully explored, unique critter species, medicinally beneficial organics, and even precious treasures have been discovered. It is rumoured Morfilods go spelunking to obtain the best pearls, Bauquemares for lost or hidden knowledge, and Ygraynes in order to practice their arcane rituals in secrecy. Even the Palace has been said to make good use of their bit of Great Labyrinth, casting the worst offenders of society into its depths to never be heard from again. And as a staple of all dark unknown quantities, the unmapped regions are apparently haunted by bilious faceless ghosts.