For thousands of years, a fist-sized octopus called the argonaut has baffled scientists. Even Aristotle wondered why this creature wraps its body in a thin "paper nautilus" to swim. Now, video evidence suggests these creatures have a highly-sophisticated ballast system.

Aristotle thought the paper nautilus (a thin jacket of skin, which you can see in the video above) was a tiny sail the octopus used to float on the water. But as Ed Yong explains:

An argonaut uses its shell to trap air from the surface and dives to a depth where the encased gas perfectly counteracts its own weight, allowing it to bob effortlessly without rising or sinking. [Octopus specialists Julian] Finn and [Mark] Norman filmed and photographed live animals in the act of trapping their air bubbles, solving a mystery that has been debated for millennia . . .
Their work on argonauts was based on observations of wild animals. They rescued three greater argonauts (Argonauta argo) from nets in the Sea of Japan, released them into Okidomari Harbour and filmed them as they adjusted to their freedom. It's their beautiful video that graces the top of this post.

All of the females were checked before their release to make sure that they had no air already trapped in their shells. Without this air, they were in danger of sinking and had trouble keeping their shells upright. All three animals fixed this problem in the same way.
Each one used its their funnel to jet to the ocean surface and bob the top of its shell in the overlying air. The shell has a couple of apertures at the top, which allows the argonaut to gulp in air, sealing it inside with a quick flick of two of its arms. Having sealed away this pocket, it points its funnel upwards, rolling the shell away from the water surface and forcing itself downwards. At the depth where this compressed bubble cancels out its weight, the argonaut levels off and starts swimming.

Bio-submarines. Just awesome.

link http://io9.com/5543167/millennia+old-octopus-mystery-is-solved