Making a Cartesian Diver

I made a Cartesian Diver by cutting the tip off a plastic dropper pipette. I then wrapped blu-tack round the open end so that it would float upright. I half filled the remains of the dropper with water and upended it into a bottle of water. What took time to perfect was getting the correct balance between the weight of blu-tack pulling downwards and the upthrust due to the air. Too much blu-tack and it sinks to the bottom and stays there; too much air and it won't go into the water. It took 10 minutes of trial and error with much emptying of the bottle to extract the diver to get it to work. Once complete, the bottle top needs to be tight on. Then squeeze the sides of the bottle and watch it sink...

The explanation is that the increased water pressure squeezes the air molecules closer together. This makes the air more dense. Things float when they are less dense than water. The careful balancing act here ensures that the extra pressure exerted is enough to increase the density enough to cause it to sink.