Big Southern Butte is a prominent mountain near the center of Idaho’s Eastern Snake River Plain, rising about 2,500 feet above the surrounding lava plain east of Craters of the Moon National Monument. The largest and youngest of three rhyolitic domes formed there over more than a million years, it is itself about 300,000 years old. Two coalesced lava domes form its broad mass, roughly 6.5 kilometers or 4 miles across at the base, with a combined volume of about 8 cubic kilometers. Its immense size, relative youth, and stark relief make it one of the region’s defining landmarks and one of the largest volcanic domes on Earth.