On a recent scuba-diving course, the instructor took a plastic bottle down to a depth of 30 metres, filled it with air from an oxygen tank and screwed the cap back on. As he predicted, back at the surface the sealed and now highly pressurised bottle seemed very light. Sure enough, when he released the air at the surface the bottle seemed heavier. Surely this was just an illusion?
The pressurised air added at depth should have added weight to the bottle, rather than reducing it. If it was an illusion, how are people so easily taken in by suggestions like this? And if it wasn’t, what was going on?