[Photo provided to China Daily]

"There was a clear puff, accompanied by a plume of dark smoke. The air, trapped inside for more than three centuries and thick with the smell of mold, charged out.

Wearing a face mask and with a rope tied to his waist, Zhao, fresh from the Archaeology Department of Peking University, was the first to get in.

"The sleeves and both parts of his trousers were sealed tightly so no noxious air could enter," Sun says.

When Zhao arrived at the stone gate standing right behind the wall, it was closed. Under the light of an electric torch, he discerned a narrow opening in between the two stone panels. Pressing himself against that opening, he could see a huge rectangular stone slab leaning against the panels from inside the chamber. It was a lock, a firm one－anyone who wished to enter the forbidden ground needed to find a way to remove the stone－from the outside.

Zhao's answer to the challenge was some thick iron wire.