Part of Serbia lands below Danube river is pretty mountainous, with complex geology especially in eastern parts where Carpathian and Balkan mountains collided and over eons formed Serbian Carpathians with total of 14 independent mountain ranges in existence today. These rocks date back to the Proterozoic Eon (2.5 billion - 541 million years ago) with limestones and dolomites mainly formed from late Jurassic to early Cretaceous, around 100 million years ago. There are dozens of large caves within these mountains and many with tourist paths built to visit and admire their beauty and history. Two of them we visited last week and they both gave us extraordinary experience and impressions. However, the first cave in this blog story belongs to the one formed in the foothill of an ancient volcano of the nowadays mountain of Bukulja in western Serbia, although the recent paper posted a theory that the mountain is much younger (15 million year ago) and instead formed in tectonic processes.