Some say that is precisely what it is — a huge pyramid built perhaps 12,000 years ago by an unknown civilisation. And yesterday they set out to prove it.

In the spring sunshine, watched by crowds of locals, journalists and the contestants in this year’s Miss Bosnia competition, a team of archaeologists began excavating the so-called Pyramid of the Sun, hoping for one of the greatest finds in modern history.

Experts ridicule the notion. “This is total nonsense. It’s impossible. There was no high culture in this region at that time capable of building something on this scale,” said Professor Enver Imamovic, a respected archeologist from Sarajevo.

But there is just enough evidence to suggest that the experts could be wrong — and to persuade Semir Osmanagic, 45, a wealthy Bosnian who moved to Houston 15 years ago, to raise £100,000 for the project.

The four sides of the Visocica hill have regular slopes of almost exactly 45 degrees, facing north, south, east and west exactly. “Nature simply cannot build such a perfect geometric shape,” Mr Osmanagic said.

Scattered in the hill’s scrubby vegetation are sandstone slabs that look as though they have been polished and cut. Running into the west side is a “causeway” 500 yards long and 40 wide.

And beneath the valley there appears to be a network of tunnels. Yesterday the diggers opened one which, according to local folklore, runs more than two miles to the “pyramid”. They penetrated far enough to find side tunnels. “This is definitely not a natural formation,” declared Nadjia Nukic, a geologist.

There is more, claims Mr Osmanagic, who sports an Indiana Jones-style cowboy hat. “Radar analysis showed there are tunnels inside and they are perfectly straight and the intersections are at 90 degrees.” Satellite thermal images indicate that the hill cools faster than its surroundings, suggesting that is less dense. There was a curious “resonance” from the hill when the town was shelled during the Bosnian war.

“We’re talking about a huge construction effort here. The size of this pyramid will shock the archaeological world. It’s substantially higher than the Great Pyramid of Egypt,” said Mr Osmanagic, who has spent 15 years researching the pyramids of Central and South America. “We have discovered tunnels linking the pyramid with other hilltops. We have also found a huge entrance plateau, paved with thousands of handmade sandstone plates. Once the world accepts that we have this huge pyramid we will start trying to answer the questions: who? when? how? and why?”

Mr Osmanagic even claims that two other smaller triangular hills in the valley are pyramids lost to history. He calls them the Pyramids of the Moon and of the Dragon. “I believe there are at least five pyramids in this valley,” he says.

The diggers will sink ten shafts. The work should last six months. Meanwhile, Visoko, a depressed town of around 40,000 inhabitants, is cashing in. Shops have begun selling pyramid T-shirts and money-boxes and triangular pizza on triangular plates. The excavation “may or may not prove that the pyramids exist, but it’s good for business”, Esref Fatic, a souvenir-shop owner, said.

But archaeologists and historians fear that a valuable site is being ruined by random excavations. On top of the hill are the ruins of a medieval Bosnian fortress built on a Roman observation post, itself built on an Illyrian settlement.

“They should not be allowed to destroy genuine sites in the pursuit of these delusions,” Garrett Fagan, a professor at Penn State University, said. “It’s as if someone were given permission to bulldoze Stonehenge to find secret chambers of lost ancient wisdom underneath.”

Monuments to the high and mighty

• The Ancient Egyptians built pyramids as burial monuments for the wealthy; more than 100 stretch from the Mediterranean to modern-day Sudan
• Many other cultures built pyramids, notably the Mayans and Aztecs of Central America. Ancient pyramidal structures have also been found in Iran, Italy, Greece, China, France and Polynesia
• The first pyramid was probably that of Hellinikon in Greece built in 2720BC
• The largest by volume is the Great Pyramid of Cholula in Mexico

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