Liquid?
Be very careful here. The history of Mars observation and imagery is littered with episodes of people interpreting liquids that in retrospect have proven to be artefacts of the telescope (or digital imaging system), or dry dust in low-lying areas that mimic the shape of a lake.
I'll happily lay a VERY large bet that this particular image is:-
1) Not a cave
2) Not a liquid
3) cold and dry