Fark linked up this Denver Post article that details how scientists finally puzzled out the purpose and use of a mysterious set of centuries-old earthworks:

The people of this high desert, without benefit of metals, wheels or written language, maintained at least four massive waterworks from A.D. 750 to 1180 to survive the devastating droughts of the Four Corners region. The last of these works studied, a large mound dubbed Box Elder Reservoir, wasn't discovered until a 2002 wildfire burned off a dense, high carpet of sagebrush.

Includes the very weird phrase "water impoundment". Makes it sound like someone was trying to jack cases of Perrier.