Lava tubes snapped snaking across Mars The New Scientist

Dramatic 3D images of ancient lava tubes on the Martian volcano, Pavonis Mons, have been captured by the Mars Express spacecraft.
Lava tubes are produced when lava on the top of a lava flow cools and forms a crust, while the subsurface lava remains molten. This molten lava continues to flow until the lava source is exhausted. In the case of Pavonis Mons, researchers believe the roofs of these tubes eventually collapsed, leaving long channels in the planet’s surface.