About 15 millions years ago Lesvos was covered by tropical and subtropical forests, a unique characteristic both for Greece and the Mediterranean. This special vegetation was due to the great water resources that the island had and continues to have till today, but mainly to the active volcanoes that existed under the ground. This tropical forests with the gigantic trees were suddenly covered by lava and volcanic ash, following an intense volcanic activity. The water running through these volcanic grounds for thousands of years crystallised the trunks of the trees. The stone trunks preserved till today, at an area of 150.000 acres on the north-west side of the island, are mainly comprised of quartz and opal, while the characteristics of the trees are clear. The Fossil Forest has been proclaimed as a Preservable Monument of Nature and visitors are free to visit the greater part of it.