Diving underwater is something that has attracted mankind since they figured out that there was a living world under the seas. SCUBA is an abbreviation for Self Contained Underwater Breathing Apparatus, in other words, equipment to allow us to breathe underwater.
The air that is in the tanks is compressed air as we know it outside, so parts nitrogen, parts oxygen and some other gases as well. Many people mistakenly believe that the air is pure oxygen, but that would actually poison the divers! Because of the nitrogen in the compressed air, divers need to be very careful when diving to greater depths, as nitrogen narcosis can occur, which has the same effect as excessive alcohol. This is easily solved by going back up to shallower waters.
A more serious effect of nitrogen is known as Caisson’s Disease also known as decompression sickness, which is when nitrogen starts to form bubbles in the diver’s body. This becomes an issue when the diver returns to the surface and normal air pressure and can lead to death. This is why one of the most important things to remember for divers is to never go too deep too quick and never to go back to the surface too quickly.