I ended up watching some of the old Seaquest DSV episodes online via Netflix this weekend. Yeah, it's corny, but Bob Ballard showed up at the end of each show to give a 30 second "how this applies to real life ocean exploration" pitch. The man has got some interesting ideas... check out his TED talk to get an overview.
In one of these talks, he mentioned that someone had recently been down to 2200 ft (presumably he meant in submersible), and that kind of caught my attention.
Recreational divers usually call it quits around 100-130 ft because after that, oxygen actually starts to become toxic at the 80% nitrogen/almost-20 oxygen mix we breathe. Going deeper means having to use a "trimix", where a 3rd gas (apparently helium) is substituted in to reduce the oxygen to nonlethal levels. The depth record for this was set at 1100ft (330m) in 2005 by Pascal Bernabé (check out his account of the trip). The next record depth requires the use of an Atmospheric Diving System, which is basically a hardsuit... 2000 ft (609m) by a Navy diver in 2007.
It all reminded me of The Abyss and how liquid breathing had been a scifi idea for a long time. The official name is perfluorocarbon, and it turns that it does indeed work and mice have been able to breathe in it for long periods of time. The downside is that it strips out another liquid that's in your lungs and you end up basically suffocating if it's ever removed. The other problem with "The Abyss" scenario is that at high pressures, other biological process in the body start to do whacky things... protiens go nuts, tissues start ripping. Not good.
So while that might cap the ability to dive to 6000m and meet alien lifeforms, the perfluorocarbons are still medically interesting in other ways. It's got potential for use as artificial blood, accelerating flushing out of nitrogen after decompression sickness (the bends), washing out the lungs, and it's currently used in eye surgery in several different ways.
That's some interesting stuff discovered for a 10 second quip about diving records. :)