THE WOODLANDS, Texas—Liquids may have pooled and flowed on Pluto’s surface within the last million years – and they may do so again, scientists reported March 21 at the Lunar and Planetary Science Conference.

Though not a given, the presence of liquids on Pluto at any point is puzzling, given that average temperatures on the frozen world hover around -400 Fahrenheit. But, as NASA’s New Horizons team is learning, Pluto is anything but dead—or predictable.

“What the data revealed did not surprise us,” says NASA’s Jim Green. “It shocked us.”

Two lines of evidence suggest the dwarf world’s surface may occasionally be just a little bit wetter than it is now. One is based on how the planet’s atmospheric pressure