The National Autonomous University of Mexico (UNAM) has its main campus, Ciudad Universitaria (University City), situated in the Coyoacán borough of the southern part of Mexico City. Built by architects Mario Pani and Enrique del Moral in the 1950s on an ancient solidified lava bed called "El Pedregal", it replaced the separate buildings in the downtown area that had been used for teaching. The entire project cost approximately $25 million and was the most expensive construction in Mexico since the Aztecs. It includes the Olympic Stadium, 40 faculties and institutes, the Cultural Center, an ecological reserve, the Central Library, and a few museums. In 2007, UNESCO declared it a World Heritage Site. UNAM has other buildings in Mexico City, as well as in other Mexican states and countries like Canada and the United States, but Ciudad Universitaria, also known as "C.U.", is the most recognizable symbol of the university.