'Stranger than any imagination could conceive'
That may be. I will only add for now that from the little I have learned a good part of that diversity is benthic and includes cold water corals living as deep as six thousand metres down. Stephen Cairns offered me a glimpse of this extraordinary world, stranger than Mars, when I sought him out in February last year deep in the stacks of the Smithsonian, thanks to an introduction from Tom Goreau.
Focussing on the Southern Ocean, Angelika Brandt of ANDEEP, which sampled 6348 metres down and discovered 585 new species of crustacean, is reported as saying "The number of species out there are certainly unknowable".
P.S. 23 May: J Murray Roberts writes: "Reef framework forming corals like Lophelia are pretty sparse around Antarctica but there are records in the Subantarctic. Cairns published a monograph on this in 1982 (Biology of the Antarctic Seas XI, Antarctic Research Series, Volume 34, Paper 1)".