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what happens to the lungs of a freediver during a deep dive, boyle´s law in action
(www.youtube.com)
When I took scuba lessons in the late 1970s, we performed a similar demonstration in reverse. We filled a balloon on the bottom of the pool, then watched it expand and finally burst as it rose to the surface.
It was a very vivid demonstration of why divers with an air supply absolutely must never hold their breath while ascending.