Lab tests have proved that bubbles can sink floating objects. The findings add weight to suggestions that methane bubbles escaping from methane reserves in the seabed might have been to blame for vessels disappearing in the Bermuda Triangle and the North Sea. The Greek mathematician Archimedes realised that for something to float, the density of the liquid has to be greater than the density of the object. So a simple argument is that if you mix enough bubbles into a liquid to lower its average density, an object floating on its surface should sink. It is suggested that this process is behind the mysterious demise of many ships that sank for no obvious reason.

However, Bruce Denardo at the Naval Postgraduate School in Monterey, California, was sceptical. He points out that rising bubbles often carry currents of water up with them, exerting an upwards force on the floating object. For all but the most violent bubbles, this upward drag might be enough to keep an object afloat. Denardo and his colleagues decided to test the theory. They filled a four-litre glass beaker with water, then fed in air at the bottom at varying speeds. Then they dropped in steel balls filled with varying amounts of water and air to see how easily they would sink. If, in the absence of rising bubbles, the ball only just floated on the surface, switching on the bubbles made it sink.

Denardo concludes that we can't rule out the methane theory for ships lost in the Bermuda Triangle. "If a phenomenon can be made to occur in a lab, it probably occurs somewhere in the natural universe," he says. Journal reference: American Journal of Physics (vol 69, p 1064)