Based on the Great Blue Hole (Belize) and Dean's Blue Hole (Bahamas), mixed with the Bermuda Triangle.

These are the entrances to the plane of monsters, and while they generally remain in their general area, they can open and close about.

The area is covered in a thick fog, and the waves will easily range from heavy and dangerous to deadly still.

More recently, zombi ships will also emerge from this area. (Though, zombi ships will attack monsters before attacking humans.)

It is unsafe, so ships will avoid it at all costs unless to foolishly hunt monsters; the rare ships that survive are left with an incurable curse.

Oton was born here