Mars Canyons (image) ETH Zurich Caption A pit chain marks a subterranean lava tunnel. Its roof collapsed partially. Credit Image: Mars Image Explorer / asu.edu Usage Restrictions Copyrighted by Mars Image Explorer / asu.edu Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.