VISOKO, Bosnia-Herzegovina -- Researchers on Wednesday unearthed geometrically cut stone slabs that they said could form part of the sloping surface of what they believe is an ancient pyramid lying beneath a huge hill.
Archaeologists and other experts began digging at this central Bosnian town last week to explore the team leader's theory that the 2,120-foot hill covers a step pyramid, which would be the first ever found in Europe.
"These are the first uncovered walls of the pyramid," Semir Osmanagic, a Bosnian archaeologist who studied the pyramids of Latin America for 15 years, said of the stonework found Wednesday.
"We can see the surface is perfectly flat. This is the crucial material proof that we are talking pyramids," he said.
Osmanagic believes the structure will prove to be 722 feet high, or a third taller than Egypt's Great Pyramid of Giza.
The huge stone blocks discovered Wednesday appeared to be cut in cubes and polished.
"It is so obvious that the top of the blocks, the surface is man-made," Osmanagic said.
Earlier research on the hill, known as Visocica, found that it has 45-degree slopes pointing toward the cardinal points and a flat top. Under layers of dirt, workers discovered a paved entrance plateau, entrances to tunnels and large stone blocks.
Satellite photographs and thermal imaging revealed two other, smaller pyramid-shaped hills in the Visoko Valley.
Last week's excavations began with a team of rescue workers from a nearby coal mine being sent into a tunnel believed to be part of an underground network connecting the three pyramid-shaped hills.
They were followed by archeologists, geologists and other experts who emerged from the tunnel later to declare that it was certainly man-made.
The work at Visoko, about 20 miles northwest of Sarajevo, will continue for about six months. Two experts from Egypt are due to join the team in mid-May.
"It will be a very exciting archaeological spring and summer," Osmanagic said.