Were there once oceans on Mars? Scientists have found what they believe to have been a vast Martian coastline. From Slate Magazine:

Was This Once a Vast Martian Coastline?

Red Planet Riviera: Ancient Mars Ocean Found?

Aeolis Dorsa is a vast plain about 1,000 kilometers (620 miles) east of Gale Crater, where NASA’s rover Curiosity is currently exploring. Using high-resolution observations from NASA’s Mars Reconnaissance Orbiter’s High-Resolution Imaging Science Experiment (HiRISE) camera, the Caltech team spotted what appears to be a river delta leading to a large depression, a candidate for the basin of an ancient ocean.

“Scientists have long hypothesized that the northern lowlands of Mars are a dried-up ocean bottom, but no one yet has found the smoking gun,” said Mike Lamb, assistant professor of geology at Caltech and a co-investigator of this research, in a news release. The research has been published in the Journal of Geophysical Research.

It is thought that large regions of the Red Planet’s northern hemisphere was once covered in water as the majority of the landmass is at a lower elevation than the southern hemisphere. This is exactly where you’d expect to see evidence of ancient large bodies of water. But over the aeons, the evidence has weathered away, making any positive identification of such features difficult.