Why did the Aztecs build their capital on a swampy island in a lake?

The Aztecs built their capital on a swampy island because the better land was already taken. The people we call the Aztecs, who called themselves the Mexica, were latecomers to the Valley of Mexico, where many city-states were already established. They settled on unpromising islets in Lake Texcoco, and from that muddy start they went on to dominate much of Mesoamerica.

The Mexica told it differently. According to their own legend, in 1323 they were shown a vision of an eagle perched on a prickly pear cactus, eating a snake, and the vision showed them where to build. In the prophecy, the cactus had grown from the heart of Copil. Whatever really happened, the image stuck: today an eagle on a nopal cactus sits on Mexico's flag and coat of arms.

The island was far from an obvious choice. But the Mexica were not deterred by the unfavourable terrain. They farmed with chinampas, wrongly nicknamed floating gardens, to dry out and expand the island, and they linked their city to the shore with causeways whose bridges could be pulled away to protect it.

The result was astonishing. By 1519, when the Spanish arrived, Tenochtitlan is estimated to have held 200,000 to 400,000 people. Among European cities, only Paris, Venice and Constantinople might have rivaled it. Its ruins now lie beneath the historic center of Mexico City.