Crumbling ruins have been discovered dotted all over the area near the famous Angkor Wat temple in Cambodia.
Experts believe there were about 1,000 temples there at the height of the Khmer empire in the 12th century.
Angkor Wat, which has become Cambodia's main tourist destination, is the sprawling centre piece in the area.
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Researchers have always suspected there had to be more than just temples out there buried beneath the jungle.
Dr Peter Sharrock, art historian, said: "The tropical forest grows so fast and so dense that we've no idea what was there in the Mayan civilisation of central America and the same goes for South East Asia."
He added: "We've got years of work ahead of us now with the new maps that have been provided."
The man who led the discovery is Australian archaeologist, Damian Evans, who recently presented the findings to a media conference in London.
Four years ago they conducted an aerial survey using lasers and found evidence of a large city.
The team also discovered other, apparently sophisticated, densely populated cities that would have been connected by roads and canals.
Life is vastly different today, quiet and rural, but there's still a connection to the past. Those who make a living here hope the new findings will to more tourism and more development.
Reoun Savon, a carver in the area, said: "If they can discover new things like temples it will be good for us because more people will come. It will be good for business, more tourists mean more money."
The findings may also have far-reaching implications on the history of the entire region.
Speaking on Al Jazeera Wayne Hay can be seen exploring the remains of what used to be capitals of the Khmer Empire.
He said: "The new discoveries aren't visible to the naked eye but they are leading researchers to the conclusion that, at one stage, the Khmer Empire was the largest in the world."
Some time in the 15th century it declined, most believe it came after an invasion which forced Angkorians to flee but the new discoveries hidden beneath the trees may change our understanding of what happened there all those years ago.