Part 5. In 1519, Hernán Cortés wrote a letter to the King of Spain describing the main market of Tenochtitlan as "twice as large as that of the city of Salamanca," with 60,000 people buying and selling there every day. Salamanca was one of the largest cities in Spain. Cortés had spent years in cities. He had never written a letter like this one. Tenochtitlan had an estimated 200,000 people when Cortés arrived. London at the time had around 70,000. Rome had roughly 50,000. The city was built on an island in Lake Texcoco, connected to the mainland by three massive causeways, each wide enough for ten horses side by side. A twin-piped aqueduct carried fresh water from Chapultepec Hill into the city, one pipe in use while the other was cleaned. The streets were swept daily. Spanish soldiers wrote home about how clean they were. Tenochtitlan had public toilets at intervals on the causeways and inside the market. Feeding 200,000 people on a lake required a solution no European city had attempted. The Aztecs engineered chinampas: permanent agricultural plots built directly into the shallow parts of Lake Texcoco, constructed from layers of lake mud and organic matter, separated by narrow canals that kept the soil irrigated year-round. Because crops sat above lake water permanently, the growing season never stopped. Well-managed chinampas could produce up to seven harvests annually. Their yields ran as high as 13 times more food per hectare than European grain farming of the same era. The chinampas of Xochimilco, in the southern part of what is now Mexico City, are still working today and are a UNESCO World Heritage Site. Cortés destroyed Tenochtitlan in 1521. The Spanish then drained Lake Texcoco over the following centuries and built Mexico City on top of the ruins, using the drained lake clay as foundation. That clay has been compacting ever since. In 2026, NASA satellite data confirmed Mexico City is sinking by roughly 10 inches per year on average, with some areas moving faster. Over the past century, the ground has dropped nearly 40 feet in total. The Metropolitan Cathedral, construction begun in 1573 directly on the site of the Aztec ceremonial center, now visibly tilts to one side from the uneven subsidence. The main city square, the Zócalo, sits below the elevation of where the lake's surface once was. The Spanish built a city on a lake they drained. The lake is still taking it back. Cortés wrote to the Spanish king that the market at Tenochtitlan sold goods from across the known world. The city that held that market is still sinking into the lake that fed it.