The floor inside the imposing Temple of the Inscriptions at Palenque is covered by massive slabs of limestone. The highly-regarded Mexican archaeologist, Alberto Ruz, was curious about one slab which had rows of holes with removable stone plugs.

Removal of the plugs enable Ruz's workmen to pry out the slab (1949).
Underneath was a secret passage leading down; it had been tightly
packed with rocks and rubble to block access.

Over the course of four seasons, 400 tons of this "fill" was
laboriously removed, until a room was reached at about the level of the
base of the temple.

In this anteroom were found the skeletons of several young adults, ritually
sacrificed to accompany Lord Pacal on his journey into the Underworld.
At the far end of the room, passage was blocked by a large stone slab.

Removing this wall, the archaeologists discovered the vaulted funerary
chamber, 9 metres long and 7 metres high. Stalactites and stalagmites
had formed in the room over the centuries since Pacal's corpse had been
sealed inside.

Dominating the crypt was a huge sarcophagus, covered with a heavy stone
lid on whose surface was carved a superb work of art. The lid was
carefully raised using automobile jacks.