The Mexican Plateau covering most of north-central Mexico is a rugged, arid-to-semiarid tableland with an average elevation of 6000 feet (1825 meters) above sea level. This photograph reveals the southern end of this large plateau with its highest elevation of near 8000 feet (2435 meters). The Mexico City Basin is immediately west of three large, north-south-oriented volcanoes (center of the photograph). Snow-covered at its 17 887-foot (5450-meter) summit, Popocatepetl Volcano is the southernmost and highest of these three volcanoes. The single volcano east of this trio is Malinche Volcano, with the rim of its caldera measuring in excess of 14 600 feet (4450 meters) above sea level. The major air stagnation and pollution problems that plague the Mexico City Basin are easily understandable given the physiography of mountainous terrain and volcanic peaks encircling three-quarters of the basin and the massive population within. In l994, Mexico City had a population of more than 22 million, making it the second most populous metropolitan area on Earth. Some demographers predict that Mexico City will have 40 to 50 million people by 2010. With its current rate of growth, Mexico City will surpass the greater Tokyo metroplex as the world’s most populated urban area by 2000.