>>10944153Methane, I believe.
There has been some speculation that certain lost ships may have sunk because large deposits of methane broke loose, bubbled to the surface, and because the water is being aerated it fucks buoyancy up and the boat would just sink like a rock to the bottom of the sea before anyone knew what was happening.
Pretty spoopy stuff, but I don't know that there has ever been any corroboration to any of it.