It's not the work of aliens. Instead, you can chalk these crop circles up to humans + money + time. And, with the help of satellite imaging, you can watch as humans use money to change the desert over the course of almost 30 years.
Landsat is a United States satellite program that's been in operation since 1972. Eight different satellites (three of them still up there and functioning) have gathered images from all over the world for decades. This data is used to help scientists studying agriculture, geology, and forestry. It's also been used for surveillance and disaster relief.
Now, at Google, you can look at images taken from eight different sites between 1984 and 2012 and and watch as people change the face of the planet. In one set of images, you can watch agriculture emerge from the deserts of Saudi Arabia — little green polka-dots of irrigation popping up against a vast swath of tan. In another se, you'll see the deforestation of the Amazon. A third, the growth of Las Vegas. It's a fascinating view of how we shape the world around us, in massive ways, over a relatively short period of time.
Maggie Koerth-Baker is the science editor at BoingBoing.net. She writes a monthly column for The New York Times Magazine and is the author of Before the Lights Go Out, a book about electricity, infrastructure, and the future of energy. You can find Maggie on Twitter and Facebook.