Mars Rover Opportunity seems to have roved into a rather deep patch of sand from which it is now having trouble extricating itself. Check out this hazcam photo to see just how far into the Martian sand the wheels have sunk. The rover team at JPL is carefully working on solutions for Opportunity to wiggle and ease out of the sand without getting more stuck.
Another advantage for human explorers: an astronaut could simply lift his knees higher.
Also check out Spirit rover’s lovely panorama from the slopes of the Columbia Hills.