If the air in an upside-down container (such as a diving bell) weighs much less than water, then why does it not get compressed down until it is the same mass per volume, when submerged even to a shallow depth? For example, as I understand it (though admittedly I am not an expert), a diving bell filled with air at 1 atmosphere (1 bar), when dragged down to a certain depth in the ocean, can be, say, several hundred times denser than it originally was. But air that is 300 times as dense is still far LESS heavy per volume than water. Yet, if kept at the same spot, it stays that way and doesn't get more compressed, even though it is lighter, correct?
What other forces make that happen?
Also, does the afore described process ALSO happen with other liquids i.e. would a diving bell work in, say, liquid hydrogen or liquid helium (possibly with a gas other than air, one that has a much lower freezing point)?