The Lop Nur Potash Ponds are located in the Taklamakan Desert of northern China. Built and expanded over the last 20 years, the ponds occupy the basin of a dried up salt lake that once spanned 10,000 square kilometers (3,900 square miles) and left behind salt crusts as thick as 100 centimeters (39 inches). The variety of color seen here illustrates the stages of production—water is first dyed blue to absorb more sunlight and heat, and it gradually fades in color as it evaporates and potash crystallizes, ready for extraction.