Darkchannels

The Darkchannels are a world-spanning maze of underground rivers, oceans, caves, passageways, nooks and crannies.
It is near impossible to traverse the Darkchannels without hitting dead ends, passages which turn into underwater tunnels, and vast lakes which require a vessel to cross safely.
The Dro'zu have called the Darkchannels their home for millennia and as such they have developed the ability to navigate without light by using sonar, go without oxygen for hours and to last for weeks without consuming any food. This makes them well adapted to the environment, and as such they have become the dominate race of the Darkchannels.
There are many encampments and a handful of cities within the Darkchannels, but locating them is quite difficult for anyone that is not extremely familiar with the environment.
The Drain is a massive whirlpool which funnels directly into the Darkchannels and thus there are many areas which are constantly flooding with powerful streams of water which burst into the caverns directly from the ocean. No one quite knows when The Drain first came into existence but it is a constant danger as new fissures appear often, splitting open cavern walls, caving in ceilings and turning settlements into lakes.