"Only when the drinking of tea thrived during the Tang Dynasty (618-907) was the old trail reactivated and became a major commercial route between the East and the West," says Chen, 67, who since the 1990 journey has never let up on his research.

He describes the route as a "life road" because the Tibetans living at high altitudes acquired a taste for tea in the Tang Dynasty, and it became a staple of their daily life.

However, because those mountainous areas are not conducive to tea growing, the long-distance tea trade began from Yunnan where tea trees were largely cultivated.

Tea produced in Yunnan was carried on the backs of horses, mules and even yaks into high-altitude areas, and to the south, including to what is now India, Laos, Myanmar, Thailand and Vietnam. It was also taken to the north, through the Xinjiang Uygur or Inner Mongolia autonomous region, to Russia.

"Russia is very cold and high-altitude," Chen says.

"Tea grown in China was largely transported to Russia using the ancient route."

Tea trees require a certain altitude, sunshine, humidity and particular soil types to grow, and those requirements kept the ancient road humming with trade for centuries.