Lost Forest Research Natural Area is a protected relic forest closely associated with the Christmas Valley Sand Dunes in northeastern Lake County. Fossil Lake is an area nearby where many fossils of prehistoric animals have been identified. Glass Buttes is an obsidian complex in the extreme northeast corner of the county. Crack in the Ground, northeast of Christmas Valley is a long fissure with ice in its floor year round. Big Hole, Hole-in-the-Ground, and Fort Rock are ancient maar craters in the northwestern part of the county.