Occupying the entire northeastern section of Guatemala, the Petén is the country's largest and least populated province. Most of the Petén is forest -- thick tropical rainforest -- and its lush and wild landscape contains some of Mesoamerica's richest archaeological treasures. In 1990, the government of Guatemala officially established the Maya Biosphere Reserve, a tract of 1 million hectares (2.5 million acres) that includes most of Petén province. The Maya Biosphere Reserve adjoins the neighboring Calakmul Biosphere Reserve in Mexico and the Río Bravo Conservation Area in Belize, comprising a joint protected area of more than 2 million hectares (5 million acres). The Petén Province is home to perhaps the most impressive and best preserved of all ancient Maya ceremonial cities, Tikal, as well as other less excavated sites, including Yaxhá, El Ceibal, El Mirador, and Uaxactún. The area is also a rich and rewarding destination for bird-watchers and ecotourists.