Huge stones that make up parts of China's Forbidden City were transported along artificial ice paths lubricated with water, a team says.
That's despite the fact that wheeled vehicles had been developed 3,000 years earlier.
The colossal city was built in the 15th Century by workers at the start of the Ming dynasty.
Writing in PNAS journal, the team says that wood-on-ice sliding was more reliable than using wheels.
Using sledges on ice was much more expensive and needed more workers but it was deemed a safer method than mule-powered transport.
Howard Stone, professor of mechanical and aerospace engineering at Princeton University, said that on his first tour of the Forbidden City, he and colleagues were curious about how some of the huge stones had been transported.
