These pictures introduce the basic features of a martian gully. The figure on the left is an example from Mars, the figure on the right is a gully on Earth. In the Earth picture, rain water flowing under and seeping along the base of a recently-deposited volcanic ash layer has created the gully. For Mars, water is not actually seen but is inferred from the landforms and their similarity to examples on Earth.
JPL/NASA/Malin Space Science Systems
There is good circumstantial evidence that abundant underground ice and, possibly, liquid water exist on Mars.
