Ok so you'd think after 2 parts, you couldn't get much more deeper than almost the bottom of the ocean.

BUT YOU'D BE WRONG

You see, life has a funny way of figuring things out. Currently, it's nigh on impossible to really get a proper grip on this life, because to date there has only been two manned deep-sea vehicles (DSVs) that could get down there, one in the 1950's, the Trieste, and a fairly modern one, Deepsea Challenger which David Cameron famously piloted.

There are of course some unmanned machines, but even that's tricky. Recently in 2008, the Japan Agency for Marine-Earth Science and Technology lost the only one capable of researching these depths due to a cable snapping!

The only reliable method left is the rather brutal dredging of the deep seas which just pulls anything up and damages the ecosystem in the process.

With that in mind, let's continue from the previous post without the hope that I'll provide any real answers, and instead take a look at those bizarre sea cucumbers, arthropods and other crazy critters, because there's a lot more down there than you might expect. There's also a harrowing message at the end for you to enjoy.

Biodiversity

Regardless of how fish manage to stay alive so deep, it seems that at a certain point, life ceases to function. except for the arthropods, worms, anemones, foraminifera, echinoderms, bacteria...Ok that's still quite a lot.

But that's not even the end, recent dives from the NOAA Ship Okeanos Explorer have discovered organisms professionally named 'strange, feathery-wispy things' and 'strange little spheres'. Things that make a whole room silent, dumbfounded.

In a paper looking at the biodiversity around the Marianas and Guam, it's noted that 5,640 species were recorded in marine habitats around the Marianas, making the place the most biodiverse marine environment of this size anywhere on earth. So it's no surprise that every single dive leads to nothing but new discoveries. The paper also points out that the high rate of endemism (a state of having lots of unique life) found deep within the trench is likely a result of its total isolation and stability for millions of years. Still, it doesn't explain their survival so seriously don't ask, just check these out:

Acorn Worms

Acorn worms are apparently quite abundant, but a specific new family of them, Torquaratoridae, have been found quite recently which limit themselves to the deep sea. Usually, acorn worms live within sediment on the seabed, but these new ones actually crawl along the surface and float along in water columns... at about 3 inches per hour. In the meantime, a perpetual trail of feces follows them. Nice.

Arthropods

Amphipods, crustaceans and the like seem to be the most numerous complex creatures to be able to survive under such incredible pressure. What's cooler is that they are often victim to deep-sea gigantism, growing over 20 times the size of the shallow-sea equivalents. And... yes, nobody knows how they survive down there.

Pigs

These sea pigs, or scotoplanes are unique among the sea cucumbers in that they have tiny little legs that they use to walk along the ocean floor. They live primarily off dead stuff it sucks up from the sediment.

Bacteria

Bacterial extremophiles surviving in insane conditions, have been known for quite some time, from as far back as the 50's. Back in 1968, researchers found 31 species in sediment from a 10,400 metre deep collection. Many were not endemic, however, giving the organisms the adjective 'barotolerant' - Sure, they can live down here if they want, but if they feel like going up to the surface, well, why the hell not?

Those which can only thrive down there (what a life choice) are called Obligate barophiles. This includes species of the previously mentioned xenophyophores. Obligate barophiles, aside from falling apart when being brought up to land from the change in pressure, lack the components within their DNA to protect them from UV radiation, so they are somewhat forced to remain down in the darkness which also makes them psychrophilic (read that carefully) - cold lovers.

There is a little information about how these guys survive, mostly relating to a high percentage of unsaturated fats as previously mentioned, and some structural differences to their proteins and enzymes being folded differently to normal in a way that is affected less by the heavy pressure.

This is particularly intriguing because genes related to these deep sea adaptations have also been found at atmospheric pressure, which leads some to believe life originated in the deep sea.

The bacteria thrive in various ways, including chemosynthesis, but they have largely been found feeding off the organic detritus floating down from the surface, called marine snow. They therefore play a crucial role in the overall carbon cycle of life.

Unfortunately, the most isolated place on earth does not escape the grubby hands of humanity.

Recent researchers looking at various crustaceans at the bottom of the Marianas Trench, 11,000 metres down found traces of plastic; Nylon, PVC, PVA and more in their stomachs.

What more can I say?

Main image sources
other images CC0 Licensed

_{Sources: Barophilic enzymes | Isolating deep sea bacteria | Piezophile | Japan losing a DSV | Marine biodiversity of Guam and the Marianas: overview | 2016 dive discoveries | More discoveries | Torquaratoridae}