An Abyss...

An Abyss can be visited by zailing above them with a zubmarine.

Description
A chasm gapes below your zubmarine, descending into unknown fathoms.

Accessibility
 appears in the captain's available event log when zailing over an abyss while traveling Under the Unterzee.

The value of Abyssal Rites: Above an Abyss (and thus the abyss type) for each abyss is randomly set for each new captain. Three of the abyss types will be accessible via four different abysses on the captain's Map, while one abyss type will be accessible from only three different abysses.