"These are the first uncovered walls of the pyramid," said Semir Osmanagic, a Bosnian archaeologist who studied the pyramids of Latin America for 15 years.
Osmanagic said Wednesday's discovery significantly bolsters his theory that the 650-meter (2,120-foot) hill rising above the small town of Visoko is actually a step pyramid -- the first found in Europe.
"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 itself is a colossal 220 meters (722 feet) high, or a third taller than Egypt's Great Pyramid of Giza.