Cartesian Diver Explained: How Pressure Controls Buoyancy

The Cartesian diver sinks when the test tube is squeezed because of the increase in pressure inside the bottle. The pressure causes the water in the bottle to rise and enter the test tube, displacing the air inside. This decrease in air volume increases the density of the diver, causing it to sink.

When the test tube is released, the pressure inside the bottle decreases and the water level drops, allowing air to re-enter the test tube. This decreases the density of the diver, causing it to float again.