Wreck diving is a specific type of scuba diving. Most wreck diving is done on shipwrecks, but wrecks of aircraft are also commonly explored by scuba divers. There are also a lesser number of more exotic wrecks which are dived upon, ranging from trains and buses to collapsed naval radar stations.
All scuba divers require some form of training, but many divers engage in additional training before diving on wrecks. Wreck diving is often subdivided into three types:
- non-penetration, ie. swimming over and around the wreck.
- limited penetration, going into an overhead environment, subject to a limit of 130 feet/40 meters cumulative linear distance to surface (ie. depth plus length of penetration).
- full penetration, going deeper into the overhead environment in the wreck.
Non-penetration diving can usually be undertaken safely by most certified scuba divers. Divers are often recommended to undertake additional training, such as a wreck diver specialty course, before engaging in limited penetration diving. Full penetration diving is regarded as a type of technical diving, which requires significant additional experience, training and equipment.
Overhead environments may prevent direct access to the surface. Sharp edges may be present on the wreck. Some wartime shipwrecks may contain explosive cargos, and some wrecks may contain toxic materials or pollutants. Wrecks are also a magnet for fisherman, due to the fish that often gather on them, and fishing line and nets may become snagged on the wreck and present an entanglement risk for divers.
Most wrecks are slowly disintegrating. The structure becomes weaker as the material corrodes or rots, and at some stage it will collapse. This is not a good time to be inside. Those wrecks which are exposed to rough sea conditions will generally deteriorate faster, but oddly enough, are also usually less of a risk for collapsing on divers, as this usually happens during a storm when no divers are around. The wrecks that are below the wave action are the ones which are more likely to collapse without warning, possibly when an unlucky diver bumps againat a critically weakened structure, and precipitates a collapse. This is very unusual, but remains a possibility, and is one of the reasons why divers should be adequately trained before attempting penetrations — it helps if you can recognise some of the signs of impending structural failure.
In conditions of poor visibility it is possible to accidentally penetrate some wrecks. If you are lucky, you will also accidentally find your way out again. If you are suitably skilled, you may also find your way out. When diving at a site where this is a possibility, it is prudent to tow a surface marker buoy on a line, or tie off a line at a point known to be outside the wreck before venturing too close in the darkness. Your line is the only guarantee of a way out.