[Photo provided to China Daily]

Today the stele, lying in a glass cabinet in one of the two exhibition halls of the mausoleum, is presented by tour guides to visitors as "the key of Dingling". And the wall mentioned is the one that separates the burial chambers of the emperor with the tunnel that leads to it.

"Elated by the new find, the team started digging its third and last tunnel from where the second stele was found, at the back of the Worldly Towers. Not long into digging, they discovered another tunnel－not a brick one but a much sturdier stone one that ran toward the center of the burial mound.

"It's clear now that the stone tunnel constitutes the last section of the journey taken by Emperor Wanli and his empresses en route to their final resting place," Yang says. "The team could not find the stone tunnel initially because there is a turn between the brick tunnel and this stone one."

At the end of this 40-meter long tunnel lies "the wall", called jin gang qiang, or "the impregnable wall". Made of big stone blocks, the wall indeed seems impregnable until the archaeologists realized that the central part of it could be easily dismantled, by pulling out the blocks of which it is composed, one by one, like pulling drawers from a chest.

Sun remembers vividly when the first stone block was pulled out.