Whether or not the Red Planet ever had water on it has long been solved. Plenty of evidence exists for ancient water flows and Mars still contains frozen ice at its poles. But a more tantalizing puzzle remains: Does liquid water still flow on the Martian surface?

Since 2011, NASA’s Mars Reconnaissance Orbiter (MRO) has been taking pictures of mysterious dark features that appear during warm seasons, flow downhill, and then vanish as the weather grows colder. Known as “recurring slope lineae” these odd streaks have become one of the hottest topics in Martian research. While its been years since their discovery, the features just keep growing more mysterious.