The massive stones used to build parts of China's Forbidden City in the 15th century were pulled by several hundred workers more than 40 miles on sledges across artificial ice, researchers say. China had discovered the wheel, but a sign at the Forbidden City hinted that the builders chose to use ice sledges because it was more reliable, even though it needed more workers. Though most scholarly work in China doesn't mention the ice transport, researchers at Princeton University found a historical document that mentioned the ice method and they made calculations about how the rocks would have been dragged along ice paths.

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