An Australian fur seal, Arctocephalus pusillus doriferus. Credit: Hanna Geeson Deep dives push marine mammals to their physical limits. During long periods underwater, animals can switch to 'anaerobic' (oxygen-less) metabolism in organs outside the heart and brain, leading to a buildup of lactic acid. Although many diving species have evolved ways to reduce the risk of 'the bends', nitrogen bubbles can still form in their bloodstream. Scientists have long believed these animals recover mainly