Sinkhole in the Greek Peloponnese. Sinkholes, or dolines, form when an underlying cave passage collapses forming a depression on the surface above. This sinkhole is about 55m on it's longest side and up to 15m deep. It lies in an area of karstified limestone. The enclosed microclimate provided by the sinkhole supports a few tall trees, a rarity in the flora of this region.