How Deep Can Seals Dive?

Seals can be found in all waters of the world, from the Arctic and Antarctic to tropical waters. They belong to the order Pinnipedia, which means "fin-footed". Pinnipeds have four flippers — one pair in front, and one pair in back. Pinnipeds are mammals — they're warm-blooded, give live birth, nurse their young, breathe air, and have hair. Since they live in the marine environment and find their food at sea, they are called marine mammals.

Most seals can stay underwater for 15 or 20 minutes before surfacing for air, and during that time they can dive several hundred feet. California sea lions have been trained to retrieve objects from depths greater than 500 feet, but the champion diver is the Weddell seal of the Antarctic. Weddell seals have remained underwater for nearly an hour, and pressure gauges attached to them have recorded dives exceeding 1,800 feet.

Being mammals, seals need air; they cannot take in oxygen from the water. Seal blood has about five times as much oxygen-carrying hemoglobin as human blood; that means the animals take a large amount of oxygen with them when they dive. During dives, the heart rate slows to 15 beats a minute or fewer, significantly lowering the amount of oxygen the seal requires. The body temperature also drops. Strange as it may seem, a seal expels most of the air in its lungs before it dives. This is believed to prevent it from getting the bends.