[MHGinTN]

Layers of methane hydrates exist with huge pockets of methane gas below them. When the hydrates (the more solid congealed form of methane and salt water) break open from a slide or quake, a large volume of methane gas can be released. An oil derrick in the Gulf of Mexico sank from such a gas release, if memory serves. The buoyancy of the floating medium changes dramatically with gas mixed ture in salt water.

[marigold]

Okay, you got me curious now. Would the bubbles need to be methane, or would ordinary air bubbles work as well? Assuming there is no explosion involved.

Carting a load of air around underwater sounds safer and more environmentally-friendly than a load of methane.