Two thousand years after the Chinese began using wheeled vehicles, they turned back to a much more primitive method of transportation. In order to construct the Forbidden City, an imperial palace for the emperor, throughout the 15th and 16th centuries, workers slid giant stones—some weighing more than 300 tons—down artificial paths of ice  that extended from Dashiwo quarry, 70 kilometers southwest of the city, to the Beijing site of the new palace. A team of engineers has for the first time examined the benefits that an ice path—created by freezing water spewed onto the ground from a series of wells—would have over other transport methods. Many of the giant stones, such as the central carving shown above, were above the capacity of wheeled vehicles at the time, so a pulled sledge would have been more reliable, the researchers found. What’s more, to pull a single 123-ton stone on a dirt road would have required 1537 men, they report this week in the Proceedings of the National Academy of Sciences; to drag the same stone across frozen ice would take 338 men; and to tug the stone on ice lubricated with a thin film of fresh water: only 46 men.