Is it a bird? 🦅 Is it a plane? ✈️ What appears to be an ocean on Earth are desolate dunes on Mars. In this image acquired on Jan. 2, 2014, our Mars Reconnaissance Orbiter spotted a relatively rare class of dune fields located among canyon wall slopes. Known as “wall dune fields,” these dunes are identified as either climbing or falling. Falling dunes are defined as large bedforms with lee faces on the downhill side—indicating that this is the direction of their migration—and on moderate slopes. On Earth and Mars, these types of dunes are largely controlled by what is called “microtopography.” Obstacles can change airflow, create turbulence, potentially enhancing erosion, deposition, and/or transport of dune sediment.