pyro; dolphins the best free divers in the world...? I'd agree they are one of the most graceful and benevolent creatures in the ocean but in terms of depths reached I believe the sperm whale carries the gong on that one. evidence suggests the most ridiculous depths around 1000m, possibly further. thats 1 kilometer down...think of the huge pressure on the whale that does that! and holding their breath the whole time is staggering, they manage this by shutting off various parts of their circulatory system leaving only vital systems in supply...for example dolphins may drop heart rate from 100-90 down to around 20. quite astonishing diving mammals, I'm fascinated by deep diving animals.