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 AS. scolu, sceolu, a company, multitude, crowd, akin to OS. skola; probably originally, a division, and akin to Icel. skilja to part, divide. Related to Skill, School. of fishes.
1. A sandbank in a stretch of water that is visible at low tide.
2. A stretch of shallow water; SYN. shallow.