at the gas panel to provide an adequate but noisy free-flow supply. This is normally sufficient to ventilate the casualty for the return to the surface or bell. In some cases it may be necessary to disconnect the casualty from their umbilical, if it is irretrievably trapped. This requires a few minutes work with spanners, and will lose any voice communications that may have remained. It is possible to connect a replacement umbilical if one is available. This is not usually the case from a bell, as the umbilicals occupy too much space to carry a spare. Freeing the trapped casualty Divers may become trapped in fishing nets; monofilament is almost invisible underwater. Loose ropes and lines are also an entanglement hazard; normal diving equipment has many inaccessible snag points that can trap the diver, particularly when components are left to dangle, and when clips are used which can hook onto line without active intervention by the diver (known to technical divers as suicide clips). Another entrapment risk occurs when divers try to squeeze through small gaps where they or their equipment can become wedged or caught. Old ferrous shipwrecks can be structurally unstable; they may retain their shape but have lost their strength through corrosion and therefore have components or cargos that have high potential energy due to gravity, and may collapse without warning. Divers routinely carry a knife, line cutter, scissors or shears to free themselves from ropes, lines and nets. Lifting bags can be used to help move heavy objects underwater, but are not carried as standard equipment by most divers. Taking the casualty to a place of safety The safest place may be the surface, where the supply of breathing gas is unlimited, or if the diver was diving from a bell, the diving bell. where the gas supply is relatively secure, and some first aid is possible. The procedure for retrieving the casualty to the bell is relatively straightforward. The casualty is simply carried there by the rescuer. A rescue tether may be used to aid this process as it allows the rescuer the use of both hands. Once at the bell the casualty will generally be suspended by the harness using a lifting tackle provided tor this purpose. A closed bell may be partly flooded to aid lifting the casualty through the bottom lock opening, after which it will be blown down with breathing