The BBC reports:
The monolith and an altar, dating from the 15th Century, were unearthed in the very heart of the busy capital city.
The city’s mayor described the discovery as the biggest in almost three decades.
A figure representing the rain god Tlaloc and another unidentified figure are carved into a frieze on the altar.
Under the surface
“It is a very important discovery, the biggest we have made in 28 years.
The stone slab is some 3.5m (11ft) in height and much of it remains buried beneath the surface.
Archaeologists say they think it might be part of an entrance to an underground chamber.
The ancient Aztecs began the construction of the Templo Mayor temple in 1375.
It was discovered, by accident, in 1978 when electricity workers came across a vast carving of an Aztec goddess.
Aztec mathematics: here.