Water may pile up to hundreds of kg by being propped up by mere mg of gas in a pocket, even if the gas above is only 100-150g in density but is a different type of gas than the pocket. Note that small amounts of gas will dislodge small amounts of water, but no amount of water seems to dislodge any gas pocket if the surrounding gas is a different type of gas.
Steps to Reproduce
A configuration of water pouring into a shallow trough of algae terrariums should do it. See attached screenshots and save file. Terrariums will eventually be cleaned and drop polluted water, which will emit a polluted oxygen pocket. When the terrarium water storage is full, the water will start piling up to hundreds of kg even if the propping gas pockets are mere mg, while the pure oxygen above at <200g will prevent the pockets from being dislodged by the water.
This issue has not been confirmed by a developer yet.