Mexico's ancient city guards its secrets but excavation reveals new mysteries

An eight-year project at Teotihuacin, once the western hemisphere's largest city, failed to locate its rulers' tomb but findings offered tantalising clues to its origins

For decades, the hunt for a royal tomb at the ancient Mexican city of Teotihuacin has gripped archaeologists trying to unravel the secrets of the kingdom's extraordinary political power.

It is a mystery investigators thought they were on the verge of solving in 2015, when large quantities of liquid mercury were found amid a treasure trove of precious artefacts in a secret tunnel.