The meaning of term deep diving depends on the level of the diver's diver training, diving equipment, breathing gas and surface support:

• in recreational diving, 30 metres / 100 feet may be a deep dive
• in technical diving, 60 metres / 200 feet may be a deep dive
• in surface supplied diving, 100 metres / 330 feet may be a deep dive

There are particular problems associated with deep dives:

• nitrogen narcosis is related to pressure. This causes stress and inefficient thinking in the diver. When breathing air many divers find 40 metres a safe maximum depth.
• the need to do decompression stops increases with depth. A diver at 6 metres may be able to dive for several hours without needing to do decompression stops. At depths greater than 40 metres, diver may have only a few minutes before decompression stops are needed. In the event of an emergency the diver cannot make an immediate ascent to the surface without risking decompression sickness. The diver needs a disciplined approach to planning and conducting dives and needs to carry extra gas for the decompression stops.
• if long decompression stops are carried out in a tidal current, the divers may drift away from their boat cover. Diving shots, decompression trapeze and decompression buoys can help prevent this.
• gas consumption is proportional to pressure - so at 50 metres (6 bar) and diver breathes 6 times as much as on the surface (1 bar). Heavy physical exertion consumes even more gas. The diver needs to carry extra gas for deep dives.
• gas becomes denser and the effort required to breath increases with depth.
• the risk of carbon dioxide poisoning increases with depth.