There was a television program about this a couple of years ago. I can't remember if it was Nova or something like Unsolved Mysteries, but I'm pretty sure it was Nova. They went into the matter fairly extensively, saying they knew for sure that there were these kinds of deposits on the ocean floor and they do indeed let go from time to time. Although they didn't draw any definite conclusions, they did do a tank experiment with a model ship, releasing a quantity of gas from the bottom of the tank—just a sort of foam of bubbles, not a single giant bubble or big bubbles in general. The model lost buoyancy and sank. Most persuasive. This, of course, raised a lot of speculation about places like the Bermuda Triangle. Fascinating!   

Mass extinctions? OY!

Don Firth