In one breath (theoretically) a man can dive to the bottom of the ocean and come back up because the air would compress and decompress to the capacity of his lungs. But when breathing through a scuba tank if they were to go to the bottom and try to hold their breath on the way up thier lungs would burst from the expanding oxygen that was compressed. I think I saw a training where divers go to the bottom of a deep pool, and shoot back up and they have to exhale the whole time to prevent "lung over expansion injury"