I have a feeling we don't know as much as we should about the life at the bottom of the oceans. About 70.8% of the surface of the Earth is covered by water and even if we know the topology of the bottom and the currents quite good from all satellites and sonar scans, most of it is still unchartered when it comes to creatures and plants. The discovery of the black smokers in 1977 near the Galápagos Islands showed that there are some very odd echo systems down there.
The problem deep down under water, like at 2000m - 2500m down where most black smokers have been found, is the lack of energy. No sunshine reaches these depths so the plants and creatures needs to use other sources to get their energy from. The strange environment with the darkness and high pressure gives rise to truly weird lifeforms.
A good example is a snail found in 2001 around some black smokers in the Indian Ocean. It uses Pyrite (aka Fools Gold) and Greigite to build a metallic shell. Covered with an iron plate mail it's like a miniature knight in armor. National Geographic did a nice article on this gastropod and Anders Warén, the lead researcher on this metal wearing snail, says they have found several thousand more new species around this kind of hydrothermal vents, and that's just counting mollusks.
The deepest dive ever made was done in 1960 by Jacques Piccard and Don Walsh with the bathyscaphe Trieste. They reached the bottom of the Challenger Deep in the Mariana Trench spending 20 minutes at the bottom. At the depth of 10916 m the pressure is extreme, about 1250kg per square centimeter, and currently no vessel exists that can withstand it.
The Japanese DSV Shinkai 6500 is currently the deepest-diving manned submersible in the world with a maximum depth of 6500m. It was put into service in 1989, 18 years ago, but it's still new compared the next best one, United States Alvin (the one used to find the black smokers). Alvin was commissioned in 1964, but has been updated several times, last time in 2001. It can stay at a depth of 4500m for 9 hours with three people on board.
Good to see that the National Science Foundation is well on the way to a new improved 6,500m capable deep diving vehicle that can reach more than 99% of the sea floor. I'm sure it will find plenty of interesting things.