A 12 mile wide liquid water lake has been discovered on Mars by a radar mapper on the European Space Agency's Mars Express orbiter. It survives because it's capped by a layer of frozen ice with the liquid underneath. It might be very salty though, since that's apparently part of what keeps it from freezing.
https://www.yahoo.com/news/first-lake-li...22689.html
https://www.yahoo.com/news/first-lake-li...22689.html