Mexico City is the capital and largest city in Mexico. It is also the heart of the country's government, business and culture. Built on the ruins of Tenochtitlan, the ancient capital of the Aztec empire, the historic city is a mix of old and new. Skyscrapers tower over colonial churches and buildings. The famous Metropolitan Cathedral (pictured) is located on the town's main square. Construction began in 1573. It took almost 300 years to complete. As a result, the church is a blend of architectural styles. Despite its beauty, the busy metropolis faces many problems, too. With more than 20 million people living in the region, the city is clogged with traffic and pollution. (JEREMY WOODHOUSE—GETTY IMAGES)