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)

ETYM Latin chasma, Greek, from chainein to gape, to open wide. Related to Chaos.
A deep opening in the earth's surface.