LOS ANGELES (AP) — For the first time, scientists peering below the surface of Mars have detected a maze of channels apparently created by past flooding.

Such geologic features are easily spotted on the Martian surface, but researchers have not been able to find them underground until now.

Using a ground-piercing radar sensor aboard the Mars Reconnaissance Orbiter, a team of scientists led by the Smithsonian Institution made a 3-D map of the plains along the Martian equator and the channels below the surface.

The findings were reported online Thursday in the journal Science. They suggest evidence of past flooding at a time when Mars was mostly thought to be dry and cold.

Scientists say the buried channels were not detected before because the region had been covered over by lava flow.