THE GREAT WALL OF CHINA (China)
Actually a series of overlapping walls, it protected the northern borders of China from "barbarian" invaders; some entered anyway and founded dynasties (Yuan, Qing). Total length was around 12,000 miles. What we see today is mostly rammed earth; brick and stone portions have been salvaged for building materials. And no, it cannot be seen from the moon.
(Image courtesy Wikimedia Commons; CTTO)
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