In 1532, Francisco Pizarro led an expedition of battle-hardened Spanish soldiers on a fateful journey, from the desert coast of northern Peru to the highland Inca city of Cajamarca. A civil war had just ended in the Inca Empire, and Pizarro and a party of fewer than 200 men marched eastward to capitalize on the turmoil.

The ensuing Spanish conquest of the Inca had a profound effect on the region’s indigenous people, but a new paper published today in the Proceedings of the National Academy of Sciences reveals that it also had an unexpected impact on the land itself. Before the Spaniards arrived, inhabitants of the arid northern Peruvian coast clad massive sand dune–like ridges with an accidental form of “armor”: millions of discarded mollusk shells, which protected the ridges from erosion for nearly 4700 years and produced a vast corrugated landscape that “is visible from space,” says archaeologist Dan Sandweiss of the University of Maine, Orono, one of the paper’s authors.