This short dive reduces the volume of the gas in the bottles to slightly less than half the original volume. The pressure must have doubled from one atmosphere to slightly less than 2 atm. This would be the pressure at about 12 meters or 40 feet under water.
Before becoming certified to use SCUBA equipment, a course in gas laws is required.
Organizations certify divers by training them about gas laws: PADI and NAUI
This do-it-yourself Cartesian Diver allows you to replicate the dive without getting quite as wet. You also can make one using an unopend ketchup pack.
"Cartesian Demon", that has been around for several hundred years.