Machu Picchu’s terraces hide an extensive drainage system. Beneath the soil, layers of stone, gravel and sandy material help carry rainfall away. Researchers documented 129 urban drainage outlets and estimated that roughly 60% of the Inca construction effort was below ground.

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Machu Picchu gets more than six feet of rain a year. So what stops this city from sliding down the mountain? The answer is hidden beneath those green terraces. Excavations found layers of stones, gravel, and sandy material beneath the soil. Rain could drain through them instead of building up behind the walls. Across the built-up area, researchers documented 129 drainage outlets, including openings through stone walls. Those terraces did more than grow crops. They also protected the slopes from erosion. In a 1999 study, engineers estimated roughly sixty percent of the construction effort went underground. The famous stonework is what you photograph. The engineering you can’t see is a major reason it’s still there. Follow for more.