Mexico City

19.4326°N, 99.1332°W.

The largest city in North America, built directly on top of the Aztec capital Tenochtitlan after the Spanish razed it in 1521. Tenochtitlan itself was founded in 1325 on a marshy island in Lake Texcoco, expanded outward by causeways and chinampas (floating gardens) until it housed an estimated 200,000 people — larger than any contemporary European city. The Templo Mayor at the city's ceremonial center was a twin-staired pyramid 60 metres tall; its excavated foundations now lie 2 metres below the modern Zocalo. Mexico City sinks at a rate of up to 50 cm per year as the lakebed clay below it consolidates under the weight of a 22-million-person megacity. The Spanish cathedral built atop the Templo Mayor visibly tilts.

The conquest filled the lake, but the city sinks back toward it — and the older pyramid keeps reappearing under the cathedral floor.