The Challenger Deep is possibly the deepest part of the ocean. To explain why we have not found more creatures down there, let's start by asking the question, "what is it like in the Challenger Deep?". The Challenger Deep is so deep that no sunlight reaches there -- it is completely black. This means that no plants can survive, because there is no sunshine (they cannot get sunlight for photosynthesis). This explains why no plants have been found.

Next, let's think about temperature. No sunshine gets down to the Challenger Deep, so there is also no warmth. The temperature of the water is around 0-4 degrees Celsius (32-39 F), all year round. We also have to imagine the amount of pressure. This trench is so deep, that the weight of all the water above is applying pressure. Imagine when you dive in a pool, and all the pressure hits your ears and you have to pop them. Now imagine this feeling times 1000.

All in all, to survive in the Challenger Deep, a creature would have to survive with no light, cold water, and very high pressure. But it is amazing that there ARE some creatures that survive down there! They just are different kinds of creatures. On expeditions into the Challenger Deep, scientists collected samples of water and earth floor to see what was living in it. It was full of bacteria.

The last thing I'd like to point out is that because of the very high pressures, it's very hard to send down cameras, equipment or people. There have been very few times when scientists could go down and investigate -- but it's possible that there are many creatures living down there, and we just haven't discovered them yet!

On one hand, there are plenty of creatures that do live in Challenger Deep, including hundreds of microorganisms, xenophyophores (giant single-celled organisms that look like amoebas), amphipods (like shrimp), and sea cucumbers, and even the Mariana snailfish (though this isn't at the very bottom of the Deep). However, much of the "typical" ocean life - such as fish, plants, and whales - is absent. This is essentially because the conditions of deepest parts of Challenger Deep, and even other less-deep parts of the ocean, do not permit these types of life. At those depths, light is insufficient for photosynthesis. This means that plants cannot survive. The lack of plants restricts food/energy sources for other organisms, and hence the organisms which would eat those, and so on up the food chain. Certain microorganisms can survive on chemical energy taken from compounds released by formations called hydrothermal vents (locations where material from within the crust is expelled).

The extreme pressure and low temperatures also restrict the life which can survive those depths. The pressure is more than 1000 times that felt at sea level, and the physiology of most animals simply can't support it. The temperatures can be as low as ~33F. This will solidify the fats in many animals, so organisms living at these temperatures need to produce special unsaturated fats which remain fluid. The depth also limits the oxygen available (there isn't a source of oxygen nearby).

Sources:
LiveScience,
IFLS,
BBC,
NOAA1,
NOAA2,
SFTS

Hi Mario, that’s a great question! Scientists say we know more about the moon than we do about the depths of our ocean. Crazy, right? The ocean floor is hard to study because the farther you go down, the higher the pressure. At the bottom, the water pressure is 1000 times greater than the pressure at the beach. That makes it really hard for people to go down there and study it. However, we have created special vehicles that can withstand that pressure.

Because of that high pressure, it also makes it really hard for creatures to live down there. For example, the hard shells that surround some small water organisms dissolve faster in pressurized water. They haven’t found any big organisms that can dwell at those depths. Instead, they’ve spotted a sea cucumber, a scale worm, and a shrimp!

Few animals live in Challenger Deep because there is little food down there. As well, Challenger Deep is difficult for scientists to get to, so we haven't explored it fully either and there may be animals that we haven't found yet.