“I was on something like page 16 of Google search and found a laser survey done by a Mexican organisation for environmental monitoring,” explains Luke Auld-Thomas, a PhD student at Tulane university in the US.

It was a Lidar survey, a remote sensing technique which fires thousands of laser pulses from a plane and maps objects below using the time the signal takes to return.

But when Mr Auld-Thomas processed the data with methods used by archaeologists, he saw what others had missed - a huge ancient city which may have been home to 30-50,000 people at its peak from 750 to 850 AD.