Built in the 16th century by the Spanish on the ruins of Tenochtitlan, the old Aztec capital, Mexico City is now one of the world's largest and most densely populated cities. It has five Aztec temples, the ruins of which have been identified, a cathedral (the largest on the continent) and some fine 19th- and 20th-century public buildings such as the Palacio de las Bellas Artes. Xochimilco lies 28 km south of Mexico City. With its network of canals and artificial islands, it testifies to the efforts of the Aztec people to build a habitat in the midst of an unfavourable environment. Its characteristic urban and rural structures, built since the 16th century and during the colonial period; have been preserved in an exceptional manner.