The Horse Lava Tube System (or Horse system) is a series of lava tubes within Deschutes County, Oregon, of the United States. The system starts within the Deschutes National Forest on the northern flank of Newberry Volcano and heads north into and near the city of Bend.^[1][2] The system continues north again to Redmond at the Redmond Caves and into the Redmond Canyon, where the last known segment is known to exist; however, the basalt flow that created the system goes beyond to Crooked River Ranch and terminates just short of the Crooked River Gorge.^[2][3] The lava flow that created the Horse system is also referred to as the Horse Cave lobe^[4]and it filled the ancient channel of the Deschutes River which at that time flowed around the east side of Pilot Butte.^[2] The Horse Cave lobe is a part of the basalt of Lava Top Butte which also consists of the Arnold Lava Tube System, the Badlands rootless shield, and the Lava Top butte basalt.^[4][5] All have a geologic age around 80,000 years old.^[5]

• http://en.wikipedia.org/wiki/Horse Lava Tube System