Today's Image of Mars shows a lava flow on Olympus Mons' northwestern side. This is just one example of the many lava flows on the solar system's largest volcano. Flows like these occur when lava pushes through the ground and leaves a path behind it. Many of the lava flows on Mars are currently inactive and thus leave behind hollow tubes like this one, though some are still active
This image links to the original image from HiRISE, though they have not currently provided a caption.