New divers will usually feel a mixture of excitement and fear at the thought of doing a deep dive. Deep diving can certainly be exciting and it's definitely healthy to maintain a certain level of caution as well.
How Deep is Deep?
Different divers have different ideas about when a dive is considered a deep dive. To put it in perspective, an Open Water Diver is certified to dive to 60 feet / 18 meters and an Advanced Open Water diver is certified to dive to 100 feet / 30 meters. As part of the Advanced Open Water course a student will complete a Deep Dive to 100 feet / 30 meters, so for an Advanced Open Water Diver any depth greater than 60 feet / 18 meters could be called deep. The limit of recreational diving is considered to be 140 feet / 40 meters and this is the depth that a diver trained in deep diving is certified to descend to. Usually, a deep dive is considered to be a dive between 100 feet / 30 meters and 140 feet / 40 meters.
Why Dive So Deep?
The main reason to dive deep is to see things that you can't see at shallower depths. It's quite common for well preserved wrecks to be found in deeper water, as the greater depth means less exposure to surface surge. You will also find that different marine life exists at different depths. On tropical reefs it's common to find healthier coral at greater depths due to less exposure to the sun and to divers. Many fish and other marine creatures also prefer greater depths. Of course a disadvantage of diving deeper is less visibility and color due to less sunlight. Many divers will carry a dive light to bring the color back to coral and it is necessary to use strobe lighting for photography at any depth greater than 15 feet / 5 meters and particularly on deep dives.
Deep Diving Concerns
Like most types of recreational diving, deep diving is very safe as long as the proper precautions are taken. The main concerns in deep diving are increased chances of decompression sickness, rapid air consumption, and nitrogen narcosis.
Due to increased pressure at greater depths the chances of decompression sickness are increased. This can be countered by properly planning the dive using dive tables or a dive computer and ensuring that you ascend slowly and complete all necessary safety or decompression stops. Some divers believe performing deep stops in addition to a normal 3 minute safety stop will decrease their chances of suffering from decompression sickness. The dive medicine community is undecided about the benefit of such stops, although they are not thought to cause any harm.
Due to more rapid air consumption at greater depths it is important to closely monitor air gauges ad to allow a greater air reserve at the end of the dive. It is also recommended to make use of a redundant air source in case you become low on air. This means either carrying an additional small cylinder of air called a pony bottle or having a drop tank available. A drop tank is an additional cylinder with an attached regulator that is hung from a rope off the dive boat. It is normally hung at 15 feet / 5 meters, so that it is easily accessible during safety stops.
The third concern when deep diving is nitrogen narcosis. The air we breathe is constituted of 79 nitrogen, an inert gas that has no effect on our bodies under normal surface pressure. However, as we descend into the water the increased pressure increases the partial pressure of the nitrogen, which means that it has the same effect as breathing greater concentrations of nitrogen. This increased nitrogen affects the synapses in our brain and brings on a feeling very similar to drunkenness. Nitrogen narcosis becomes noticeable to different people at different depths, but begins to affect most people at around 50 feet / 15 meters. The first effects are normally tingling of the fingers, followed by slow thinking, dizziness, disorientation, and impaired decision making. Most people report feeling the effects of nitrogen narcosis at depths greater than 100 feet / 30 meters. The deeper you go the greater the effects. Nitrogen narcosis poses no long term health risks and all symptoms are relieved as soon as the diver ascends. It is recommended that dive buddies monitor each other for symptoms of nitrogen narcosis and ascend to avoid severe narcosis.
Deep Diving Courses
The Advanced Open Water course includes a deep dive to 100 feet / 30 meters. Afterwards divers are able to complete a course in Deep Diving. This specialty course involved four dives of between 60 feet / 18 meters and 140 feet / 40 meters. The course covers theory including deep dive planning and nitrogen narcosis, as well as practice using pony bottles and/or drop tanks and performing deep stops. You'll normally carry out some experiments with your instructor to test for the effects of nitrogen narcosis and are almost certain to feel it during the course. After certification, divers will be certified to dive to 140 feet / 40 meters. Depths greater than this are the realm of technical diving.