For more than three decades, tens of thousands of people have lived in tents in the Smara Refugee Camp in Algeria. In October 2012, Santiago Sierra had the letters S.O.S. engraved into the desert floor outside the camp using a road grader, creating the world’s largest piece of graffiti: 5 kilometers long and 1.7 kilometers wide, with a contour extending for 37 kilometers. Sierra’s emergency call inscribed on the landscape recalls the ongoing occupation of Western Sahara by Morocco, which began in 1975 and has driven a large majority of the population into exile; most refugees have been stranded in camps in Algeria. To this day, Morocco, backed by France, has ignored UN resolutions and rulings of the International Court of Justice demanding the return of the occupied territory to the region’s tribes. The refugees will manifestly not be able to count on earthly assistance, and so Santiago Sierra chose a size and orientation for his S.O.S. that make it seem addressed to an observer in outer space. His “world’s largest piece of graffiti” was documented in a picture taken by the satellite Ikonos III.