Magura Cave, Bulgaria
Some video clips from the Magura cave. Its pretty big one with 2.5km (1.6 mi) total length. It is in the form of a passage and when somebody enters from one end exits through the other. It is interesting because contains human remains from about 40 000 years ago and was home for some of the last Neanderthals. Also it has inside prehistoric wall paintings made of guano and carbon dated from 11 000 to 8 000 years ago. And particularly interesting for me are some of the depictions of Ostrich and Giraffe-like animals made about 11 000 years ago. I made some research and couldn't find anything like that living at the time in the area or even Europe in general. Strangely enough this issue was not researched and very few people even paid attention. I guess these that know that giant birds haven't lived among the early Europeans are different people from these that analyse ancient paintings or with other words Palaeontologists don't mix with Archaeologists. Also there are depictions of mushrooms, so the use of the magic ones is not out of question. The Sun-like paintings have corresponding number of rays and lines and suggest early attempts for creating calendars.
All that was amazing, but there were a vandalised paintings and rocks from the time when the cave was freely open for anybody. Nowadays one can enter alone after paying small fee, but generally the halls with the paintings are visited with guides.
TLDR Big cave with prehistoric wall paintings. If you visit Bulgaria one of the top 10 things to see.