People have only explored about 5% of the world's oceans, so we don't know all there is to know about them, but one of the limitations we have is the underwater pressure.
Up here, on the surface of the Earth, we have a comfortable pressure of 1 atmosphere (atm), which is the perfect amount so that we do not implode. However, every 10 meters of water depth we go down, the pressure increases by one atm. So if we go 2000 meters down, we will have 200 times the surface pressure on us.
HOWEVER, there are animals that live at around 10,000 meters below the ocean. How do they do this?
Since gas expands at that depth, the animals need to have as little of it in their bodies as possible; so lungs and swim bladders are not useful at that depth.
Since animals living there have so little gas and their bodies are made up of mostly water, they have no problem living where we would not be able to.
The deepest part of the ocean is Challenger Deep in the Mariana Trench. It is 10,984 meters deep in the Pacific Ocean.