Adjoining the Material Plane is another plane known as the Sunless Sea. It can be accessed via the “thins”, such as the whirlpool in the hidden cove on Waveless Cay.
As yet, the crew has little to no information on this place, besides that it appears to be a vast, subterranean ocean. The “sky” is a purplish haze, like deep twilight. The sea appears inky black. The one island so far discovered is covered in a forest of towering mushrooms.