Final answer:
Mars once had liquid water on its surface, with evidence such as sedimentary rocks and water erosion features. Today, evidence suggests Mars has subsurface water reservoirs that might occasionally flow on the surface, indicating a potential for habitability.
Step-by-step explanation:
Other than Earth, the inner planet that once had liquid water on its surface is Mars. There are several evidences indicating Mars's wet past, such as water erosion features and sedimentary rock formations.
Even more compelling are the gullies that imply the current presence of flowing salty water on the surface, pointing to near-surface aquifers.
Today, water can be found in subsurface reservoirs of Mars, where there are chances of habitability due to the presence of probable liquid water. Mars's surface rovers have found complexes suggesting extensive ancient surface water, and occasional water flow has been detected under present-day Mars conditions.