Image caption Workers slid massive stones along artificial ice paths, such as this 300-tonne marble carving

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.

Image caption Workers had to drag the stones about 70km the quarry (b) to the Forbidden City

"There's a story on a sign that comments about an ice pack, so we started to investigate it," he told BBC News.

"The funny thing about that sign is if you go to the leading scholarship on the science and technology of China, they don't indicate this as far as we can tell.

"Most scholarly work indicates that the Chinese had the wheel in 1500 BC and the conclusion from that was they didn't have any need for sledges dragged by man. The Forbidden City was built 3,000 years later, so you have a contradiction.

"So our suggestion is somehow that they didn't recognise the use of the artificial ice paths to drag these large rock carvings."

But when the team translated a 500-year-old document from the historical record, they found a mention of huge stone carvings transported on ice.

They then made calculations looking at the friction of ice to see how plausible it would be to drag such large rocks over the short time period in winter, when it was cold enough to do so.

Prof Stone said several hundred workers would have been needed to transport stones ranging from 100-300 tonnes.

"If you look at the frictional characteristics of ice for the rocks of this size, we estimate that 300 people were needed for this kind of dragging," he explained.

Advance planning was also vital, as water had to be laid out in the winter months so it could freeze along the 43-mile (70km) stretch from a quarry outside Beijing.

The Forbidden City Is situated in the heart of Beijing and was home to China's emperors in the Ming and Qing dynasties

Became a museum in the 1920s after the fall of the last emperor Puyi a decade earlier

Was built by more than 1,000,000 workers

"They would dig wells every kilometre, pour water on the road which would give it a very smooth surface, The ice can support a lot of weight, they were then able to drag these objects," said Prof Stone.

Other researchers say they were already aware of the use of ice to transport marble stone, but it had never been published in academic literature.

Sally Church, from the Faculty of Asian and Middle Eastern Studies at Cambridge University, had noticed the use of this method of moving marble after translating a Chinese book in 2007 called Stories of the Forbidden City. It was written in 1962, but based on earlier sources.

The text said: "Really huge stones were moved on a contraption called a 'dry ship' or 'land ship'. Large pieces of wood were put together into a raft-like object, and there were two of these.

"In very cold weather they poured water onto the roads and let it freeze, then either they or animals would pull it along."

Dr Church said that it was "certainly new" and interesting to look at this topic from an engineering perspective.

"It is just a small fact that lies buried in the sources in the subject. I would compare it to something like our knowing that the Tower of London once kept elephants and other exotic animals in a sort of zoo," she told BBC News.