The seawater in deep oceans have a density of 1050 kg/m³ or so. The density of air is about 1.3 kg/m³, about 800 times less. If air is compressed to a density higher than the water in deep oceans and released there, will it sink to the bottom then? I know that Boyle's law is not accurate at high pressures, but air compressed to 1/800 of its original volume will give the necessary density anyway. Possibly the air is not a gas at that pressure and possibly it would go in solution in the water too, but if it didn't (use a balloon?), would it then be possible to fill the bottom of a deep ocean trench somewhere with air?