Wreck diving is an exciting and adventurous form of recreational diving that involves exploring the wreckage of ships, planes, and other artificial structures. While most wreck dive sites are found in shipwrecks, there is a growing trend to sink retired ships to create artificial reef sites. Diving in a crashed plane can also be considered wreck diving. The recreation of wreck diving makes no distinction as to how the boat ended up at the bottom.
When it comes to wreck diving, there are three main types: non-wreck diving, light zone penetration, and full penetration. Non-wreck diving is the least dangerous form of wreck diving, although divers should still be aware of the entanglement risks posed by nets and fishing lines that can be caught in the wreck (wrecks are often popular fishing sites), and the underlying terrain may present a greater risk of having sharp edges. Penetration into the light zone presents greater dangers due to the height and greater proximity of the wreck structure, but due to the proximity of a visible exit point and a certain amount of external light, these hazards are more manageable. Full penetration involves the highest level of risks, including the risk of being lost inside the structure, the risk of it being completely darkened in the event of multiple power outages, and the inability to escape without assistance in the event of a total interruption of the respiratory gas supply. There are several methods for taking divers to the wreck; the preferred method will depend on local conditions.
In conditions of poor visibility, hooking up to the wreck is a reliable way to ensure that divers find it, but this procedure requires a wreck that is structurally suitable for attachment with a hook or an anchor. A shooting line that could fall between the wreckage is less likely to damage the wreckage or be stuck and difficult to recover, but this requires adequate visibility so that divers can ensure that they find the wreckage. When it is important to return to the shooting line to climb, the first divers can tie it to the wreckage using a guide line, which is retrieved by the last divers to leave. When there is a strong current, it may be necessary to fall from an upward current, a technique that is sometimes referred to as parachute jumping or free fall. The wreck may first be marked with a short line, if this is considered useful or necessary.
Divers can come to the surface on the shooting line, the anchor line or the personal decompression buoy, depending on the conditions. When using the anchor line to control ascents and descents, a dividing line can be used between the anchor line and the stern of the boat, to allow a safe transfer between these points in a stream. When sailing live, there are alternative methods of descent such as free descent and descent down the shooting line. In technical penetration diving, there are two approaches: conventional approach and progressive penetration. The conventional approach involves using continuous guidelines placed from a wreck reel tied just outside and inside entry points at regular intervals inside (to mitigate risk of cut line or linear trap).
In deeper penetrations two coils are used so that in case of total loss of visibility when diver loses contact with main line or main line is cut secondary line can be anchored and then used as reference point to sweep main line. The procedures and techniques for navigating interior of wreck using guide line are practically same as when penetrating caves. An alternative approach popularized by deep-wreck divers in Northeastern United States is known as progressive penetration which avoids use of coils but diver performs several successive penetrations each one successively deeper than previous one memorizing route for both outbound and return trip. Divers who engage in penetration diving are conventionally taught to wear three lights: main light and two backup lights which practically eliminates risk of completely losing light inside wreck. However total loss of visibility due to sediment formation remains risk. Wrecks in shallower waters tend to deteriorate faster than wrecks in deeper waters due to greater biological activity. As result many older larger shipwrecks that tend offer full penetration dives tend be deeper dives which can present additional complications; if dive in wreck is intended be decompression dive diver will normally transport decompression gases in cylinders mounted on sides but it's difficult penetrate many wrecks with cylinders mounted both on rear and on sides requiring divers use different configuration or leave their decompression gases outside wreck before penetration. This creates possibility that diver will not be able relocate their decompression gases if they leave wreck at different point from where they entered.
As long as wreck diving is not penetration diving no special wreck diving equipment is required; equipment requirements are based on situation outside wreck.