Originally Posted by 2MuchMark

A few years ago I saw a documentary about it and they suggested that there were large pockets of lighter than air gasses below the ocean floor (Methane maybe?) that would be belched up from time to time.

This would sink a ship because so much water would be displaced by the bubbles that it could just not stay afloat. If a plane happened to be fly overhead during one of these, the thinner air would cause the plane to crash.