Wreck dive

Wreck diving is recreational diving where the ship wreck,crash aircraft and other artificial structures are explored. 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.

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Types of wreck diving

• Non-penetration diving (i.e. swimming over the wreck)
• Limited penetration diving, within the "light zone"
• Full penetration diving, beyond the "light zone"

Each subsequent level has it hazards and greater risk, and normally require additional training and experience to develop the required competence, and also require additional equipment

Non-penetration wreck diving is the least hazardous, although divers still need to be aware of the entanglement risks presented which may be snagged to the wreck and the underlying terrain may present greater risk of sharp edges. A diving knife is required for this dive level.

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. there is clearly a much greater risk of entanglement and silt out inside of the structure, as well as to move laterally to a defined exit point before one can surface in the event of an emergency. however there is a low chance of panic attack in the event of oxygen content getting low. A diving knife is required for this dive level.

Full penetration involves the highest 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. imagine lost in the dark and the oxygen content getting low, there is a high chance of panic attack. A certain length of guild line are need to enter to this zone as the line is the emergency escape route.

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 (information source : https://en.wikipedia.org/wiki/Wreck_diving)