Explorers claim ruins found deep in a dense and dangerous Amazonian jungle could represent one of the great archaeological discoveries.
The multinational team has located the site in a remote region in central Ecuador.
They have already unearthed an 80m tall by 80m wide structure, made up of hundreds of two-tonne stone blocks, and believe there could be similar constructions over an area of about 2.6sq km.
Some of those involved believe the site in the Andes mountain range could be the mausoleum of Atahualpa, the last Incan emperor who was captured by the conquering Spaniards, or hold the Treasure of the Llanganates, a vast haul of gold and other riches amassed by his followers to pay for his release.
In exchange for his freedom, Atahualpa is said to have offered to fill a room with gold. But the offer was rebuffed and he was executed in 1533.
His body is said to have been exhumed, mummified and later hidden by his followers in the region in which the new site has been found.
According to legend, great treasures - which had been amassed for the ransom - were either buried with him, or separately.
Others believe the newly discovered site dates back far earlier, to unknown, pre-Inca cultures from before 500BC, citing what appear to be rudimentary tools found there.
The site is being investigated by a team of British, French, American and Ecuadorean explorers.
Among them is Bruce Fenton, an Ecuador-based Briton and researcher into the region's indigenous cultures, who has been involved in the project for about three months.
Also involved is Benoit Duverneuil, a French-American archaeologist, who undertook an expedition there earlier this year.
The vast wallslopes at a 60-degree angle, with a flat area at the top where many artefacts have been found. The team believes the summit was used for some human activities, possibly sacrifices.