Inventor help!!?

Question

im looking to build something that periodically becomes submerged but pops back to the surface seemingly of its own accord. i want it enclosed in an area probably the size of a shoe box. does anybody have any suggestions?

Answer 1

you need to use compressed air,combined with one way valves.

Answer 2

Use a pump that switches from pumping in to pumping out water and changing the buoyancy. You could use air but then you would need a snorkel.  Pump and power source must be separated from water.  Bladder?