Over four million years ago volcanic eruptions buried the trees of Lesvos in a layer of ash, after which silica-heavy water filtered through and effectively turned the trees' cellular structure to stone. It's a gripping sight, and while most of the trees are just stumps, their internal composition is beautiful to behold. The small museum attached to the forest is very well put together and offers a comprehensive background into the petrification process. Admission to the museum is about $5.
Natural History Museum of the Lesvos Petrified Forest, Lesbos, Lesbos Prefecture, Greece