The natural phenomenon of Trovants:

 stones that literally grow by themselves. Romania may be known as the land of Dracula, heaped with bloodthirsty vampires and other creatures shaped by Hollywood imagination, but it is also a land where nature has had an interesting activity. Far less known to the world than Dracula, stand an impressive number of stones which quite literally grow by themselves. The stones are real and known as Trovants.

 the stones are a type of sandstone concretion that secrete cement and can at times appear to grow as if alive.