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What set this line of mollusks apart from all that came before was a key evolutionary breakthrough, buoyancy.

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For years, its secret was a mystery, until scientists took X-rays of a living Nautilus.

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What makes Nautilus unique among the living mollusks today is that the shell is not a simple interior tube,

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but it is cut into a series of partitions.

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People began x-raying the animals, and early on it became apparent

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that not only is there air in the chambers, but there's water too.

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New chambers are sealed off with seawater still inside them.

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In order to stay buoyant as it grows,

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the nautilus must remove this seawater from its shell.

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To do so, it uses a tube called the siphoncle,

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which spirals through every chamber.

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Cells in this tube gradually pull the water into the animal's blood system, leaving only gas, the secret to maintaining buoyancy.

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500 million years ago, a buoyant shell enabled the nautiloids to grow in size and dominate the seas.