The nautilus shell is famous for its perfect spiral shape. Inside, the shell is divided into many separate chambers. As the nautilus grows, it builds a new, larger chamber to live in and seals the old ones behind it. The animal controls how much gas is in those sealed chambers to sink or rise through the water - just like a submarine using ballast tanks.
Nautiluses spend the day resting deep in the ocean where it is dark and cool, then rise towards the surface at night to hunt for small crabs and fish. They have up to 90 soft tentacles around their mouth and very large eyes, but no ink like an octopus or squid (their closest relatives).
Because the nautilus has survived almost unchanged for half a billion years while other creatures came and went, scientists call it a 'living fossil'. Palau is one of the few places where researchers regularly study nautiluses in the wild to learn more about how they live and what keeps them healthy.