Lifting bag
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A lifting bag is an item of diving equipment consisting of a robust and air-tight bag with strops, which is used to lift heavy objects underwater by means of the bag's buoyancy. The objects can either be moved underwater or to sent to the surface.
The volume of the bag determines its lifting capacity: a 100 litre / 4 cubic feet bag can lift a 100 kg / 220 pound underwater object.
The shape of the bag should distribute the volume in a vertical rather than a horizontal direction so that the open end of the bag always remains underwater. If the open end reaches the surface, air will escape from the bag and it may re-descend.