How to hide an airplane factory.

True fact: color infrared photography was developed to allow American analysts to distinguish between real vegetation and camouflage ("fake vegetation") in aerial photographs during WWII. Why was this so important? Because the Americans figured that if they could do stuff like this so could their enemies.

EDIT: California Becomes a Giant Movie Set has more details and pictures of the lengths used to disguise military operations in California during WWII. It's really interesting.