Upper Mustang was a restricted demilitarized zone until 1992. Because it was cut off from the rest of the world, it became one of the most protected places on Earth. The bulk of the population still speaks traditional Tibetan languages because the valley spans from the summit of the Annapurna Circuit south to north, all the way to the border with Tibet. The Upper Mustang Trek, and the entire Mustang region, was restricted for many years to protect the unique culture and way of life of the local people, as well as to preserve the sensitive ecosystem and archaeological sites in the area. In the past, the region saw a large influx of tourists, which had a negative impact on the environment and the local community.