Similar but smaller sangdun structures, some 2 m wide, were also found.

Wu said these structures might be from the same impressive architectural complex as the recent discoveries. He asked, "If the bigger columns indicate a main hall, is it possible that the small ones reveal the corridor?"

Xu added that a bigger map of an "underground" Forbidden City is now being drawn up through a detailed study of these scattered architectural relics.

"However, we cannot be hasty in drawing any conclusions in archaeology. It will take an extra three to five years of continuous effort in the ongoing project to comprehensively understand these layers of relics, one after another," he said.

Comparative studies of other archaeological sites can help form a "big picture" of palatial buildings during the early Ming period.

For example, Wu and several other archaeologists from the Palace Museum joined excavations at the Zhongdu ("central capital") site in Fengyang county, Anhui province. In 1369, Zhu Yuanzhang, founding emperor of the Ming Dynasty, ordered an 840,000-sq-m imperial city to be built in his hometown.

Although the plan was later halted, ruins of this half-completed prototype of the Forbidden City offer a key reference point for researchers to gain an impression of the refined nature of early Ming construction.

Sangdun equally as large as the new findings in Beijing were unearthed in Zhongdu, but they were made from gravel and earth, and relatively rougher building techniques were used.