Dig for ancient pyramid in Bosnia (BBC World News)
Semir Osmanagic is leading the project to uncover the 'pyramid'
Known as Visocica, the 650m (2,120ft) triangular mound, overlooking Visoko, has long been shrouded in local legend.
The Bosnian archaeologist leading the project says it resembles pyramid sites he has studied in Latin America.
Initial excavations have revealed a narrow entrance to what could be an underground network of tunnels.
On Friday, a team of rescue workers from a local coal mine, followed by archaeologists and geologists examined the tunnel, thought to be 2.4 miles (3.8km) long.
The team found two intersections with other tunnels leading off to the left and right.
Their conclusion was that it had to be man-made.
"This is definitely not a natural formation," said geologist Nadja Nukic.
Satellite photographs and thermal imaging revealed two other, smaller pyramid-shaped hills in the Visoko Valley, which archaeologists believe the tunnels could lead to.
Workers also discovered a paved entrance plateau and large stone blocks that could be part of a pyramid's outer surface.
Semir Osmanagic, the project leader, initially made the suggestion the Visocica hill could be a pyramid.
If he is correct, it would be the first pyramid discovered in Europe.
He has already named the three hills the pyramids of the Sun, Moon and Dragon.
Locals have begun to trade on the excitement, selling pyramid souvenirs to tourists and visitors.
The work will continue for around six months, with the first results expected in the next three weeks.
Two experts from Egypt are also due to join the team in mid-May.
According to anthropologists there is evidence of 7,000-year-old human settlements in the valley.
German archaeologists also recently found 24,000 Neolithic artefacts one metre below ground.
Mr Osmanagic says the hill is a classic example of cultures building on the top of other cultures.
The town was Bosnia's capital during the Middle Ages, and a medieval fortress used by Bosnian kings sits atop Visocica.
The fortress was built over an old Roman Empire observation post, which in turn was constructed over the ruins of an ancient settlement.