Bathyscaphe

Piccard's bathyscaphe "Trieste" set records in deep sea exploration.
This is by no means a scale model, but the similarities are there.
It is meant to be used as a cartesian diver.
For this put plastic foil or air balloon rubber over the two openings at the bottom (the ballast chutes) and secure them with O rings.
Put it in a jar which will be filled to the brim with water. close the lid tightly, trapping as little air below it as possible.
When you press on the lid, the water will compress the air inside the bathyscaphe via the membranes on the ballast chutes. Buoyancy will decrease and the bathyscaphe will sink. When pressure is reduced again, the bathyscaphe will rise again.
For this to work, the submersible must be close to neutral buoyancy, just a wee bit lighter than the water it displaces.
Adjust buoyancy by putting tiny bits of styrofoam on deck, or lead pellets in the cavity under the observation gondola.
This will take some testing!
Enjoy!