One of the often cited causes for ships that mysteriously and quickly disappear are methane bubbles, released from sub-seafloor gas pockets. The story goes that as methane rises to the surface, the bubbles cause the density of seawater to drop, and any ships in the area suddenly lose buoyancy and spontaneously sink. This effect has been describe as so powerful that it can even knock aircraft out of the sky. Is it true? Can methane bubbles really sink ships?
The short answer is yes, under very specific conditions, but not in the way proposed. At least two studies have been conducted to determine if methane can really sink ships. In a static system with no current, a bubble that rises close to a ship, but not immediately underneath it, can force the ship into a trough created by the bubble, swamp it, and cause it to sink. If multiple bubbles are present in an unconfined system, as is likely in the ocean, ships do not sink.
So while it is conceivable that some sinkings could be caused by methane gas release, the conditions are so specific that it is unlikely that this phenomenon can account for all but a handful of disappearances.
~Southern Fried Scientist
May, D., & Monaghan, J. (2003). Can a single bubble sink a ship? American Journal of Physics, 71 (9) DOI: 10.1119/1.1582187
Hueschen, M. (2010). Can bubbles sink ships? American Journal of Physics, 78 (2) DOI: 10.1119/1.3263819
I have seen a substantial methane bubble sink a party, but that was by a different mechanism.