Final answer:
Mars shows evidence of past flowing water through surface channels, dried riverbeds, and ancient lake bed remnants, but stable liquid water does not currently exist on its surface due to harsh environmental conditions.
Step-by-step explanation:
The evidence that Mars once had flowing water primarily includes geological features such as water channels on the surface, dried riverbeds, and signs of ancient lakes and glaciers.
Mars rovers like Spirit, Opportunity, and Curiosity have discovered these features, which are remnants of watercourses, dried-out lake beds, and sedimentary rocks that testify to a time when Mars had a much warmer climate, supporting liquid water on its surface. However, the presence of liquid water today on Mars is not stable due to the planet's low atmospheric pressure and temperatures, which cause water to either freeze or boil away quickly.