Archaeologists working amid the smog and din of Mexico City may be on the verge of unlocking an extraordinary time capsule.
The leaders of a team exploring a site opened up by earthquake damage believe that they have found the first tomb of an Aztec ruler.
If they are right, the site may yield one of the great treasures of antiquity, the sort of haul that fires the imagination of people far beyond academic circles.
None of the finds has been put on public display, but Britain will get an early preview.
Fourteen objects from the site will feature in the British Museum's exhibition on Moctezuma II, the last great Aztec ruler. These could prove to be the early pickings of a much richer harvest.
Colin McEwan, head of the British Museum's Americas section, said: "There is no question that this has the potential to be a once-in-a-generation find."
The dig is in the middle of what was the Aztec capital, Tenochtitlan. Near by stands the Catedral Metropolitana de la Asunción de María, which was built from the stones of Moctezuma's Templo Mayor, which was destroyed by the Spanish in 1521.
The temple's ruins were subsequently lost for nearly five centuries and discovered only by accident in 1978. Colonial buildings built around it made further exploration difficult but an earthquake in 1985 cleared the way for the present dig.
The new finds appear to be offerings left at the entrance to a tomb. Among them is a fearsome stone sculpture of Tlaltecuhtli, goddess of the Earth. Dr Lorenzo López Luján, who discovered it, thinks that it is a capstone to a burial chamber.