Plural: abysses

ETYM Latin abyssus a bottomless gulf, from Greek for bottomless; a priv. + byssos depth, bottom.
(Irregular plural: abysses).
A bottomless gulf or pit; SYN. abysm.
Any unfathomable (or apparently unfathomable) cavity or chasm or void extending below (often used figuratively)

Plural: abyss of despairs

ETYM Cf. Abysmal.
Relating to ocean depths from 2000 to 5000 meters.
Biology, more than 300 fathoms below surface of sea.

Plural: abyssal zones

The deep sea (2000 meters or more) where there is no light.
Dark ocean region 2,000–6,000 m/6,500–19,500 ft deep; temperature 4ºC/39ºF. Three-quarters of the area of the deep-ocean floor lies in the abyssal zone, which is too far from the surface for photosynthesis to take place. Some fish and crustaceans living there are blind or have their own light sources. The region above is the bathyal zone; the region below, the hadal zone.

Plural: Abyssinias

Former name of Ethiopia.
Ethiopia is a republic in northeastern Africa on the Red Sea; formerly called Abyssinia

Plural: Abyssinians

Any of a breed of medium-sized slender domestic cats of African origin with short usually brownish hair ticked with bands of darker color.
A small slender short-haired breed of African origin having brownish fur with a reddish undercoat; Also called: Abyssinian cat.

Plural: Abyssinian bananas

Large evergreen arborescent herb having huge paddle-shaped leaves and bearing inedible fruit that resemble bananas but edible young flower shoots; sometimes placed in genus Musa; Also called:Ensete ventricosum, Musa ensete.

Plural: abyssuses