When human divers come up from deep water too quickly, they get the bends, a painful, sometimes fatal result of nitrogen bubbles forming in the bloodstream, blocking the flow of blood.

Why doesn't this happen to seals, whales or dolphins?

The question that has long puzzled scientists has now been answered, at least for seals. The mechanism is probably the same for other deep-diving mammals.

When humans dive, the air pressure inside their lungs becomes greater, automatically regulated by scuba gear to balance the pressure from the water so a diver's lungs do not collapse.

When seals dive, an international team of scientists in Antarctica has found, their lungs do collapse, pushing the now-compressed air into a naturally reinforced windpipe and removing the pressurized air from the lungs, from which it would pass into the bloodstream.

When humans dive, the nitrogen in the air in the lungs goes into the bloodstream under great pressure. There is no problem as long as the pressure increases or stays high, but if the pressure drops too suddenly the dissolved nitrogen forms bubbles as quickly as does the dissolved carbon dioxide in a soft drink when the bottle cap is removed. A slow ascent gives the diver time to blow off nitrogen, which now passes back into the lungs, before the bubbles can get too big.

Because the seal's lung is collapsed, the bloodstream has only as much nitrogen as can stay dissolved at sea level air pressure. No additional nitrogen can enter. The deep-diving mammals can race back to the surface as fast as they want.