Fathom

Fathom is an underground ocean in the Underdeep. It is sometimes called the Leviathan's Eye, or just the Eye.

Reputation
To the surface world, Fathom is a bogeyman of a place, home to black markets, cut-throats and unspeakable horrors best kept out of sight. This is all true of course, but the towns at Fathom's edges are far more hospitable than those rumours would imply. Chief among these is Sundown, one of only a few places in the world fully illuminated by gaslight. It's somewhat of a local hub for various denizens of the Underdeep, making it a melting pot of ancestries less accepted in the wider world: grey folk, hollows, and even occasional humans.

Traversing Fathom is no small task. The ocean is home to so many horrors that some suspect its seafloor is a portal to the Abyss. Sailors bring back tales of sea serpents, giant squids, and mad black dragons that hide in the dark. Compasses have been known to fail, and strange fog has eaten ships whole. Not to mention the stories of Cirein-Cròin.

Loch Luim
People travel Fathom for treasure. The sea holds the ruins of an undersea city, Loch Luim, once built by Selkies – white-garbed and long-lived shapeshifters. Their whitecoats – shimmering, magical garments - fetch a high price back at shore as ingredients for magical items. More importantly, however, threads of these coats are used to make Sea Charts: magical maps which allow Fathom to be navigated consistently.