On a first-time trip to Beijing for an engineering conference last year, Howard Stone visited the Forbidden City, the seat five centuries of dynastic rule since the Ming Dynasty in the 15th century A.D., and a historical site today. After stopping by several intricately carved, massive, single-rock structures, someone in Stone's group marveled at the skill and smarts needed to haul such massive rocks into place.

"It seemed natural that when we were staring at this thing, [we] said, how in the world did it get here?" Stone, who is a professor of mechanical engineering at Princeton, told NBC News.

Ever since humans have been building large things — the pyramids at Giza, the Colosseum, Stonehenge — visiting humans, centuries later, have been puzzling at how they did it.

In 15th century China, builders relied on nature as well as a bit of their own ingenuity to construct the regal Forbidden City. Workers hauled in hundred-ton rocks into the site in the winter months not on wheeled wagons or logs, but on hardened ice paths slicked with water, Stone, who studies viscosity and how fluids interact with each other, and his colleagues confirm in a new study in the Proceedings of the National Academy of Sciences.

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For perhaps the first time since the city’s engineers drew up building plans, Stone and researchers from the University of Science and Technology in Beijing have made a case for the efficiency of these ice paths. The technique was crucial to transport the heaviest rocks which would have crushed any wheeled wagons they may have been hoisted on, Stone and co. explain. But an essential ingredient was liquid water, splashed along the path to lubricate the rock’s journey.



One 123-ton rock was brought into place from a quarry 43 miles away over 28 days, according to a 500-year old manuscript translated by Jiang Li, a professor of mechanical engineering at the University of Science and Technology in Beijing, and Stone's co-author on the new study.

Stone, Li and co. estimate that 46 men were needed to haul in the edifice in that time, on a wood sled over ice. If the sled were dragged on plain ice with no water, 338 men could do it, and if it was dragged on dry ground, a small army of 1537 men would be needed, they write.

"If you didn’t lubricate it with additional water then ... the object would have just frozen to the ground," Stone said. The text that Stone and his colleagues refer to suggests that wells were dug along the canal to bring up water to sluice the path.

He said that the surface "may be similar in a certain sense to finely constructed Roman aqueducts or some of the old Roman roads which were smooth."

In ancient Egypt, pyramid builders pushed limestone on roller logs up inclined ramps to bring them to their spot on the massive pointed structures. It’s too bad, Stone said, that "they couldn't freeze."

Jiang Lia, Haosheng Chenb, and Howard Stone are authors of "Ice lubrication for moving heavy stones to the Forbidden City in 15th- and 16th-century China" published in Proceedings of the National Academy of Sciences.

Nidhi Subbaraman writes about science and technology. You can follow her on Facebook, Twitter and Google+.