The Underdepths are one of the greatest marvels and assets for Eptan. The world spanning cave system connects every conceivable place of importance, and allows faster travel then otherwise between them. The vast caverns, pits, and tunnels criss-cross, with some being either partially or completely submerged in water. Those that possess deep enough water to allow the passage of boats and galleys, are the most famous and frequented of the under-ways. The caves themselves range from less than 30cm wide, to vast open spaces hundreds of metres across.
The ecosystems within the Underdepths are as varied as those on the surface, if not more so. They range from sulfuric ponds, to fungal caverns, from hives of disease infested monstrosities to waters that run so deep entire civilisations could be lost within them.