Secret to Building the Forbidden City? Sleds

How on earth did 15th-century laborers transport hundred-ton rocks from a quarry more than 40 miles away to the building site of China's Forbidden City? Wisely, it turns out, reports Nature. A newly translated ancient document reveals the trick: They put the slabs on wooden sleds and pushed them over the icy, winding roads. What's more, it seems they dug wells every 1,600 feet to draw up water to keep the ice slippery, explains LiveScience.

"If you didn’t lubricate it with additional water then ... the object would have just frozen to the ground," a Princeton researcher tells NBC News. He also crunches numbers to prove the value of the method: If laborers tried to use brute strength to haul the stones, it would have required 1,500 of them. With the ice method, 50 could manage it. So why not use a wheeled vehicle? China had them at that point, but they maxed out at 86-ton loads. Enter the ice. (More Forbidden City stories.)