I have received two kind PM's about where this word can be found: wwftd
hadal
of, relating to or being parts of the ocean below 6000 meters , and
Date: Tue Dec 22 00:04:21 EST 1998
Subject: A.Word.A.Day--hadal
X-Bonus: Life is an exciting business, and most exciting when it is lived for others. -Helen Adams Keller (1880-1968)

hadal (HAYD-l) adjective

Of or relating to the deepest regions of the ocean, below about 6,000
meters (20,000 feet).

[French, from Hades, from Greek Haides, the god of the netherworld and
dispenser of earthly riches, his netherworld kingdom, the abode of the
shades of the dead.]

"The bottoms beneath the high seas at depths of about 6,000 to 18,000 feet
(1,800-5,500 meters) are referred to as the abyssal region. These regions
are characterized by a temperature that seldom exceeds 4 deg. C. A second
term, the hadal region, is used in connection with the still-deeper waters
of the oceanic trenches."
Jack Pearce, Marine Biology, Colliers Encyclopedia CD-ROM, 28 Feb 1996.