Petrified Forest
The petrified forest of Lesvos was formed some 20 million years ago, when a volcanic eruption covered much of the west of the island in lava. The forest of giant redwoods and pines that then stood there was covered in volcanic ash. This had the effect of preserving the trees, which then turned literally to stone.
In Sigri you can find The Natural History Museum of the Lesvos Petrified Forest, which has lots of information about this phenomenon. From there you can then travel to the forest park, which is situated just outside Sigri.
