I read that similar "booms" occur off the coast of Chile in S. America. The local government there did a study and found that there are occasional pockets of air over the ocean that collect together and then dissipate suddenly, causing the "boom" noise. It is kind of like when you pop a balloon. But, the article said the scientists can't determine what causes these pockets of air to form. I guess none of our scientists can figure out what is going on either.