Well, the Marianas trench is the deepest of a good number of such places.
Not much of interest down there. Much much more interesting is deep sea biologics, some never getting shallower than 7000 feet. Sperm whale will dive that deep going after small squid and octopus. The deepest human have gone using perflurocarbon liquid breathing in simulation is just over 1000 feet, Yea that rat breathing liquid in the 'Abyss' movie was real. Even used in some medical extremes.
The deep stuff that fascinates me are like the sea spiders, not much unlike our terrestrial friends, but also home to several of the giant's like 40 foot squid.
We have been to the Challenger deep, 36,070 feet straight down.
Enough pressure there to crush a welding oxygen bottle.
Sort of cute little guys are the water bears/moss piggies, they can survive more than twice that depth or even the vacuum of space, now that is getting fascinating.