Two-Fluid Cartesian Diver
This Demonstration shows a 2D and 3D representation of a typical Cartesian diver in a container. The lower fluid is 11 times denser than the upper fluid. The divers have hula skirts to add flair. The "applied pressure" control is the amount of pressure you can apply to the container (up to approximately 10 times atmospheric pressure) to make it sink. The "air volume in diver" control sets the initial amount of air in the diver.
G. Planinsic, M. Kos and R. Jerman, "Two-Liquid Cartesian Diver," Physics Education, 39(1), 2004 p. 58.