Both feet...no seriously though, I've seen loons 15-feet underwater. They might go deeper.
Moose are very powerful swimmers and can dive up to 5m underwater in search for food.
Up to 100 meters which is over 300 feet deep.
50 billioon feet
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Diving and swimming underwater is very normal behavior for a 2 month old duck. Ducks dive in the wild to get food (pondweed, minnows, snails etc.). When I raised ducks for Wildlife Rescue, I let them swim in a large 3' deep childrens pool. They would dive and swim underwater for many feet. This actually strengthens their swimming skills and is a sign of a healthy duck.
Emperor penguins are known to be excellent divers, capable of reaching depths of up to 550 meters (1,800 feet). They typically dive to about 100 to 200 meters (328 to 656 feet) while foraging for food such as fish and squid. Their impressive diving abilities allow them to stay underwater for up to 20 minutes at a time.
10
10 meters
U.S. Submarines can dive 800-plus feet. (Official depth; however, can dive deeper. The US Navy itself acknowledges that most of it's attack submarines can operate down to at least 985 feet, and many to over 1400 feet.
Loihi has not killed anyone as it is still thousands of feet underwater.
5 days
As far as I know there are no underwater labs. There are aquariums that have created tanks for the study of species, and dive into ocean areas to study life using research ships ( Ballard is a good example of this), but non have built a complex underwater.