Over the last several decades, evidence has piled up that Mars once played host to liquid water on its surface. But in its current geological era, the red planet is too cold and has too little atmosphere to allow liquid to survive for long. Even at the peak of Martian summer, water would evaporate off quickly during the day, or freeze solid as soon as night hit. But that doesn’t mean it couldn’t exist beneath the surface, where pressures and temperatures might be quite different, so researchers have been looking for signs that some subterranean liquid might bubble to the surface. Now, scientists are reporting some changes on the Martian surface that seem to be best explained by a watery seep.