A dolphin carries the camera pack (Kevin McDonnell)
And it appears the animals start their dives with a few powerful strokes and then spend much of the rest of the descent in an easy glide to conserve oxygen.
The team looked at Weddell seals hunting beneath the ice in the Antarctic, a northern elephant seal diving in Monterey Bay, a bottlenose dolphin swimming off the coast of San Diego and a 100-tonne blue whale diving in waters close to northern California.
Video cameras were coupled with data from time-depth recorders, speed meters and other instruments. These revealed that all the marine mammals used the same laid-back, energy-saving approach to diving.
Most began with 30 to 200 seconds of swimming followed by a relaxed glide for the remainder of the journey downwards.
"Basically, they're turning the motor on and off in the course of a dive, and that enables them to reduce oxygen consumption by 10 to 15% compared with what they would need if they swam all the way down," said team member Professor Terrie Williams from the University of California, Santa Cruz.
A Weddell seal performed the most remarkable feat, gliding for 6.2 minutes while descending to 540 metres.
What surprised the researchers was how the animals all used common tactics to go deep, even though they are very far apart on the evolutionary tree. The animals' lungs collapse progressively with increased pressure.
"The mass of the animal remains the same while its volume decreases, so it starts to sink," Professor Williams explained.
"The progressive collapse of the lungs in marine mammals preadapts them for taking advantage of the buoyancy change. By resting on the way down, the animals are able to extend their dives.
"They're pacing themselves, saving energy and conserving oxygen until they need to expend it for hunting or avoiding predators."
The research, which is published in the journal Science, was funded by the US Office of Naval Research, the National Science Foundation, and the National Geographic Society.
