There are some places in the earth where human cannot reach due to difficulty. Among them, the underwater is most prominent. Sea excels 14.696 psi at every 32 feet depth. Scuba divers can able to dive to the depth of 65 psi(approx. 142 feet) safely. French diver Maurice Fargues recorded the capability to withstand maximum of 191 psi. But sadly, this lead to his death. Below this level leads to lose consciousness due to oxygen toxicity. At this level, oxygen is more dangerous. By blending helium with breathing gas, the divers can go little more depth. Recreational divers can be allowed to go upto 130 feet safely without the risk of nitrogen narcosis.
The quick change is pressure also important as it endangers human lives. According to BBC, four divers who went to the depth of 9 atm(atmosphere pressure) and returned to the surface are killed due to sudden change in pressure of air filled organs.
The sea is approximately maximum of 18,000 feets deeper. Comparing this, humans reached approximately 2% of the depth. Lot of surprises, adventures and wealth are below these depth. Fortunately we are having robotic eye to explore the underwater. http://www.nautiluslive.org, http://oceanexplorer.noaa.gov offers you such a great opportunity to explore the underwater world.