An air lake.

[KezzaWezza]

I had a thought and wanted to find out if it's a possible thing.

If you have a crater on Mars (or the moon) lets says 100m deep and 800m across. If you filled that crater with oxygen, would the oxygen remain in the crater ?

 

[Helio]

No. The atmosphere is ~ 95% CO2, which is a molecule heavier than O2, so the heavier molecules will ooze into the crater forcing the lighter diatomic molecule out. This ignores other factors that would also contribute to the problem, like wind.