Mesa Verde National Park in southwestern Colorado includes the most spectacular architectural ruins left behind by Native Americans in North America. The rugged terrain is dominated by mesas and canyons, and these cliff dwellings were built into shallow caves in the canyon walls by an unknown ethnic group in the 13th century, well before the arrival of Europeans. They mysteriously disappeared from the area around 1300, maybe due to the long draught at the time. Navajos who later settled in the area called them the Anasazi, or 'the ancients'.