Some of the steep canyons that fringe a methane sea on Titan, Saturn’s largest moon, are filled with liquid—and so are the tributaries of lengthy river network that flow into them, a new study suggests. Using radar data gathered by the Cassini probe during a fly-by, astronomers determined the floors of the canyons—which measure less than a kilometer wide and are up to 570 meters deep—were essentially flat, with no features larger than 2.7 millimeters high. That’s flatter than any natural feature on Earth, so it’s a strong sign the canyons are filled with liquid, the researchers propose. Plus, the level of the canyon floors matches that of the sea, another clue that the material is liquid that can flow, the team reports online this week in Geophysical Research Letters. Because the canyons are both steep-sided and broad, the surface of the sea must have been much lower at one time, only later rising to fill the river-carved gorges with liquid. Previously, it wasn’t clear whether the canyons seemed dark because they were filled with a radar-absorbing material or were being seen from an angle and therefore in shadow. Radar data gathered during the same flyby hint that several tributaries in the river network (seen flowing to the sea from the lower right corner of the image) also contain liquid, a hint—but not proof, the researchers caution—that these rivers are still actively eroding Titan’s surface.