Wreck diving

Wreck diving is recreational diving where the wreckage of ships, aircraft and other artificial structures are explored. The term is used mainly by recreational and technical divers. Professional divers, when diving on a shipwreck, generally refer to the specific task, such as salvage work, accident investigation or archaeological survey. Although most wreck dive sites are at shipwrecks, there is an increasing trend to scuttle retired ships to create artificial reef sites. Diving to crashed aircraft can also be considered wreck diving. The recreation of wreck diving makes no distinction as to how the vessel ended up on the bottom.
Some wreck diving involves penetration of the wreckage, making a direct ascent to the surface impossible for a part of the dive.

Environment

The environment of wreck diving is sunken shipwrecks and other vehicles and structures, either sunk by misfortune, acts of war, or intentionally, as targets in military exercises, to serve as artificial reefs, or as recreational dive sites for the diving tourism industry. Wrecks sunk as tourist dive sites are usually cleaned up and have many hazards removed before scuttling, and are usually sunk in a place where the other natural hazards are not too serious. Wrecks sunk through misadventure tend to be in places with a wider range of environmental hazards, in many cases contributing to the wrecking event. In many cases the wreck itself presents a range of hazrds, such as entanglement, sharp edged metal plating, disorienting interiors, unstable structure and loose silt deposits.

Reasons for diving wrecks

A shipwreck may be attractive to divers for several reasons:

it serves as an artificial reef, which creates a habitat for many types of marine life
it often is a large structure with many interesting parts and machinery, which is not normally accessible to casual observers on working, floating vessels
it often has an interesting history
it presents new skill challenges for scuba divers to manage the risks associated with wreck penetration.
it is part of the underwater cultural heritage and may be an important archaeological resource
it provides a first-hand insight into context for the loss, such as causal connections, geographical associations, trade patterns and many other areas, providing a microcosm of our maritime heritage and maritime history
it may contain artifacts of historical, artistic and/or monetary value, which can be recovered for profit or collection purposes.
Types of wreck diving

In The Advanced Wreck Diving Handbook, Gary Gentile sub-divides wreck diving into three categories:

Non-penetration diving
Limited penetration diving, within the "light zone", the parts of the interior of the wreck from which light entering the wreck from an exit can be seen. There may be parts of the interior illuminated by exterior light penetrating though holes too small to exit through, and these are not generally considered part of the light zone, as they cannot be used as an escape route. Limited penetration limits penetration to where the diver can see the way out.
Full penetration diving, beyond the "light zone"

Each subsequent level involves additional hazards and greater risk, and therefore will normally require additional learning and experience to develop the required competence, and may also require additional equipment and the competence to use it effectively.
Non-penetration wreck diving is the least hazardous form of wreck diving, although divers still need to be aware of the entanglement risks presented by fishing nets and fishing lines which may be snagged to the wreck, and the underlying terrain may present greater risk of sharp edges.
Penetration within the light zone presents greater hazards due to overhead and greater proximity of the wreck's structure, but because of the proximity of a visible exit point, and some amount of external light, those hazards are more manageable. However, there is clearly a much greater risk of entanglement and silt out inside of the structure, as well as the requirement to move laterally to a defined exit point before one can surface in the event of an emergency.
Full penetration involves the greatest level of risks, including the risk of getting lost within the structure, the risk of complete darkness in the event of multiple light failures, and the inability to escape unassisted in the event of a complete disruption to breathing gas supply.
These categorisations broadly coincide with the traditional division between "recreational" wreck diving and "technical" wreck diving.

Procedures and safety

Wrecks may present a variety of site-specific hazards to divers. Wrecks are often fouled by fishing lines or nets and the structure may be fragile and break without notice. Penetration diving, where the diver enters a shipwreck, is an activity exposing the diver to hazards of getting lost, entrapment and consequently running out of breathing gas. Management of these risks requires special skills and equipment. Many attractive or well preserved wrecks are in deeper water requiring deep diving precautions. Training agencies recommend that at least one cutting device be carried in case the diver is entangled with fishing lines, nets or ropes and to have a spare light source in case the primary light fails. If penetrating a wreck, a guideline tied off before entering a wreck and run out inside the wreck is required by training agencies. A guideline can help a diver to find the way out more easily in case of low visibility, and has often been necessary for survival. For penetration diving, a reserve of breathing gas sufficient to allow the diver to exit the wreck and make a safe ascent in the event of any reasonably foreseeable single failure of equipment is required by training agencies' protocols and scientific codes of practice. Many wreck divers use a minimum of the rule-of-thirds for gas management. This allows for 1/3 of the gas down and into the wreck, 1/3 for exit and ascent and 1/3 reserve. In dives where decompression stops are required, this may not be sufficient. In addition, because of the potential fragility of the wreck, the likelihood of disturbing sediments or disturbing the many marine animals that take advantage of the artificial habitat offered by the wreck, extra care is required when moving and finning. Many divers are taught to use alternative finning methods such as frog kick or modified flutter kick which direct the thrust of the fins away from the bottom where most of the silt is likely to deposit. Good buoyancy control is necessary for safe and non-destructive diving in the environment of a wreck.
Image:shipwreckporthole.jpg|frame|Diver with porthole recovered from a shipwreck in New York's Wreck Alley

Connecting to the wreck

There are several methods for getting the divers to the wreck. The preferred method will depend on local conditions. In low visibility, hooking onto the wreck is a reliable way of ensuring the divers will find it, but this procedure requires a wreck that is structurally suitable for snagging with a grapnel or anchor. A shotline which can be dropped off the wreckage is less likely to damage the wreck or become snagged and difficult to retrieve, but this requires appropriate visibility for the divers to be sure of finding the wreckage. When it is important to get back to the shotline for ascent, it may be tied to the wreckage by the first divers on site using a guide-line, which is retrieved by the last divers to leave. When there is a strong current, it may be necessary to drop in from up-current, a technique sometimes known as "parachuting in" or "free drop". The wreck may be first marked with a shotline, if this is considered useful or necessary. Divers may surface on the shotline, anchorline or personal decompression buoy depending on the conditions.
When using the anchor line to control ascents and descents, a "tag line" may be used between the anchor line and the stern of the vessel, to allow secure transfer between these points in a current. When there are alternative methods for descent, including free drop and descent on the shotline.

Penetration navigation

In technical penetration diving, there are broadly two approaches.

Use of guidelines

The conventional approach involves the use of continuous guidelines laid from a wreck reel, tied just outside the entrance point, just inside the entrance point, and at intervals inside. In deeper penetrations, two reels are used, so that in the event of a total loss of visibility where the diver loses contact with the primary line or the primary line gets cut, the secondary line can be anchored and then used as a reference point to sweep for the primary line. Procedures and techniques for navigation inside the wreck using a guide line are very much the same as in cave penetration.

Progressive penetration

An alternative approach, popularised to a limited extent by deep wreck divers in the American Northeast, is referred to as "progressive penetration". Progressive penetration eschews the use of guidelines, but the diver makes several successive penetrations, each deeper than the last, memorising the layout for both the inward and outward journeys. The method is vulnerable to complete loss of visibility in a silt-out, and any disorientation in an unfamiliar area or due to nitrogen narcosis. It relies on accurate recall where an error can be fatal, and where a more reliable option is easily and affordably available. As a navigational technique, progressive penetration is generally considered unsafe. As a surveying technique it tends to be inaccurate unless measurements are also recorded. It is not taught by any of the mainstream recreational diver training agencies.
Divers engaging in penetration diving are conventionally taught to carry three lights - a primary light and two backup lights - thereby virtually eliminating the risk of completely losing light inside the wreck. Nonetheless, total loss of visibility due to a silt-out remains a risk.