Natural gases built up under the sand? Like the small gas bubbles that can build in aquarium sand only on a larger scale? I think I saw something about a lake somewhere that had this mysterious underwater "explosions"....I will have to see if I can find it....
Edit: found what I had read about but this is different. This was more gases in the water. Not necessarily the sand or lake bottom like I was thinking....
http://www.usgs.gov/blogs/features/u...n-west-africa/