Abyssal

of or like an ; immeasurable; unfathomable.
of or relating to the biogeographic zone of the ocean bottom between the bathyal and hadal zones: from depths of approximately 13,000 to 21,000 feet (4000 to 6500 meters).
Historical Examples

In their morphological characters they gradually approach the pelagic forms upwards and the abyssal downwards.
Report on the Radiolaria Collected by H.M.S. Challenger During the Years 1873-1876, First Part: Porulosa (Spumellaria and Acantharia) Ernst Haeckel

Dimya; recent in abyssal depths and fossil since the Jurassic.
Encyclopaedia Britannica, 11th Edition, Volume 16, Slice 1 Various

Such a rock is necessarily oceanic; but more than this, it is abyssal.
The Story of the Earth and Man J. W. Dawson

No other mountains in the world are at once so continuously lofty and so near a coast which drops off to abyssal depths.
The Andes of Southern Peru Isaiah Bowman

They are the summits of very lofty mountains which have their bases in an abyssal ocean.
Encyclopaedia Britannica, 11th Edition, Volume 17, Slice 3 Various

Some of the limestones are made of Foraminifera, together with Radiolaria, and indicate a subsidence to abyssal depths.
Encyclopaedia Britannica, 11th Edition, Volume 15, Slice 2 Various

Oceanic islands are those, usually very small, found in the midst of the ocean, with abyssal depth all about.
Evolution Joseph Le Conte

All are dark in color, brown, blue or violet marking the abyssal species.
Zoology: The Science of Animal Life Ernest Ingersoll

When Dr. Morton said there were abyssal fish in the lagoon, I knew why they’d be there.
Creatures of the Abyss Murray Leinster

The known deposits all point to shallow waters of epicontinental seas; no abyssal formations have been recognized.
Encyclopaedia Britannica, 11th Edition, Volume 8, Slice 3 Various

adjective
of or belonging to the ocean depths, esp below 2000 metres (6500 feet): abyssal zone
(geology) another word for plutonic
adj.

1690s, used especially of the zone of ocean water below 300 fathoms, from abyss + -al (1). Though the 19th century, abysmal was more common in oceanography.
abyssal
(ə-bĭs’əl)

Relating to the greatest depths of the oceans and to the organisms that live there. The abyssal zone encompasses all depths below 4,000 m (13,120 ft).

Relating to the region of the ocean bottom between the bathyal and hadal zones, from depths of approximately 2,000 to 6,000 m (6,560 to 19,680 ft). The abyssal zone includes nearly two-thirds of the Earth’s surface.