A Nautilus is a cephalopod (a type of mollusk) that lives inside a hard, multi-chambered shell. This unique creature lives in one chamber while the others function like the ballast tanks of a submarine. The Nautilus expels water by pulling its body into the chamber while seawater is flowing through it, creating jet propulsion that allows it to move backwards or make turns in the water. These impressive creatures also suck water into or dispel it out of smaller chambers in their shells in order to control their buoyancy, allowing for up and down motion.