To make sure the "leave no trace" rule is upheld, Burning Man relies on a Playa Restoration Team (The "playa" is the land used during the festival).
Crews of people determine which areas have the most trash - or, as Burners call it, "matter out of place," or MOOP.
They color-code the areas based on severity, green being the lightest MOOP areas, yellow being moderate, and red being the most moopy - Burning Man's words, not mine - and have a line of people clean up each area.
Then the transformation back into an ordinary desert is complete.