Tumulus Vratnica is a large prehistoric tumulus located in the Visoko municipality of Bosnia and Herzegovina. It is situated on a hill overlooking the town of Vratnica and is considered one of the largest and most impressive tumuli in the region. It is 32 meters high. It was built with a combination of megalithic double-layered blocks, clay, and concrete. The materials are concentrically stacked on top of each other. The mass of megalithic blocks reaches ten tons or more. According to georadar surveys by German geophysicists in 2007, the existence of underground passages was confirmed. Analysis of concrete material samples in France in 2009 showed the presence of geopolymeric cement as a binding material. In 2010, a team of Croatian physicists detected the presence of an electromagnetic phenomenon – an energy beam at the top of the tumulus with a frequency of 28 kHz. The existence of energetic phenomena at this prehistoric construction object has become a tourist attraction.
