The Three Sisters are a complex volcano of three volcanic peaks of the Cascade Volcanic Arc and the Cascade Range in the U.S. state of Oregon. Each exceeding in elevation, they are the third, fourth, and fifth highest peaks in the state of Oregon, and are located in the Three Sisters Wilderness, about south of the nearest town of Sisters. Diverse species of flora and fauna inhabit the area on and around the mountains, which is subject to frequent snowfall, occasional rain, and extreme temperature differences between seasons. The mountains, particularly South Sister, are popular for climbing and scrambling. Although they are often grouped together and seen as one unit, the three mountains evolved under differing geologic situations, and the petrologic composition of each mountain can vary significantly. Whereas North Sister is extinct and Middle Sister is inactive, last erupted about 2,000 years ago and still could erupt, threatening life within the region. After satellite imagery detected uplift near in 2000, the United States Geological Survey made plans to improve monitoring in the immediate area.