Lift Bag

An open-bottomed air bag used to raise objects from the bottom, filled with gas at depth and vented carefully during ascent.

A lift bag is the tool for bringing something heavy up from the bottom: an anchor, a piece of equipment, or salvage. The bag is open at the bottom, you rig it to the load with the line and clips, then add gas from an alternate air source until the object just becomes buoyant. Bags are rated by lift capacity, from around 10 kg pocket models to commercial bags lifting hundreds of kilograms.

The physics demands respect. Boyle's law means the gas in the bag expands as it rises, increasing lift and accelerating the ascent, so an unmanaged bag can rocket to the surface, spill its air, and send the load crashing back down. The technique is to add the minimum gas, control the ascent by venting through the open bottom or a dump valve, and never position yourself under the load or attach it to your body.

Proper lift bag work is taught in search and recovery and technical courses. For marking position or normal ascents, use a DSMB instead: lift bags are for loads, not for signaling.