One of the greatest thrills you can get in diving is going on a deep dive. Strictly speaking, anything in the 30 meter depth range is regarded as a deep dive. There are two ways to do a deep dive. One is on the normal compressed air tank, and the other is on tri-mix a mixture of three gases which allows the diver more bottom time without going into decompression.
Let us take a look at both. With compressed air diving, it is important to have a dive computer. The deeper you go, the less time you are allowed to spend under water. Air is made up of mostly nitrogen (the bulk of it), oxygen and rare gases. The pressure from the water will cause nitrogen to build up in the blood stream and if you stay down too long, the concentration gets higher in the bloodstream.
If that happens, once you surface, the nitrogen will turn to bubbles which get stuck in the joints and bloodstream. It is called “the bends” or “decompression sickness”. Strictly speaking, divers should avoid decompression time, which involves gradually ascending to shallower depths to allow the nitrogen to dissipate. Accidents do happen and dive computers, although right most of the time, are not always 100 % accurate. It is always better to err on the side of caution and ascend to the surface at least five minutes before the dive clock tells you to do so. It is also a good idea to do a safety stop a few meters beneath the surface as a precaution.
To do tri-mix dives, you need another certification. It is a mixture of gases that allows nitrogen to break down quicker under water and reduces the concentration of gas in the blood. It allows for longer bottom time, but if a diver stays down too long, they can get too much carbon dioxide in their blood.