The Hadal Abysses

Poul Anderson's vocabulary continues to surpass mine. On p. 175 of Murder Bound (New York, 1962), we find "...the hadal abysses." I had never come across "hadal" before and had to google it although the context had enabled me to guess its derivation from "Hades." Thus, I expect that it is pronounced something like "haydle" rather than "had-al" ?

When the private detective, Trygve Yamamura, goes to sea, Anderson is not content to describe the view from the deck or through a porthole: the sky, the weather and the surface of the sea. Instead, Yamamura's "...primitive awe..." (ibid.) inspires him to contemplate the (to coin a slightly less obscure adjective) Hadean abysses. (Other Anderson characters venture bodily into those deeps in his historical fantasy novel, The Merman's Children.)

Yamamura imagines:

"...sunless cold silence" (ibid.);
pressure that would crush the steel ship;
"...beings that were little more than glowing heads..." (ibid.);
thousand year old squids that have continued to grow all their lives;
closer to the surface, the sea serpent, a hypothetical "...great plesiosaur-like mammal..." (p. 176);
porpoises possibly as intelligent as human beings;
ocean currents potentially shifting to bring an ice age;
plankton either feeding future mankind or surviving all land life after a nuclear war.

Again, Anderson summarizes potential material for several more novels. When Poe, Wells and Doyle wrote science fiction, parts of the sea and the upper atmosphere remained unexplored and were possibly inhabited by beings as exotic as extraterrestrials. Anderson recaptures the feeling of that earlier speculation about the Terrestrial environment.