The first thing to understand about Mexico City is that it should not be there. The original Aztec capital of Tenochtitlan was built on a small island in the middle of a shallow lake called Lake Texcoco, which sat in a closed mountain basin at 2,240 metres elevation. There was no outlet for the water. The Spanish, after conquering and razing Tenochtitlan in 1521, decided to build their colonial capital on the same site rather than on the firm ground around the lake's edges. To make that work they spent four centuries draining the lake. The drainage was largely complete by the late twentieth century, and the modern city of 22 million people now sprawls across the dry lakebed and into the surrounding hills, in a basin from which water still has no natural way to escape. The clay-rich former lakebed is gradually compacting under the weight of the city, and parts of the historic centre are sinking at rates of up to 40 centimetres per year. The colonial-era Cathedral of Mexico City, built directly on the ruins of the Aztec Templo Mayor, has been visibly tilting since the 1500s.
The Geography of the Basin
The Valley of Mexico is a high-altitude closed basin formed by volcanic activity along the central Mexican plateau. It is surrounded on three sides by mountains — the Sierra Madre Occidental to the west, the Trans-Mexican Volcanic Belt including the still-active Popocatépetl and Iztaccíhuatl volcanoes to the south and east. The basin floor sits between roughly 2,200 and 2,300 metres above sea level. The original lake system consisted of five interconnected shallow lakes — Texcoco, Xochimilco, Chalco, Xaltocan, and Zumpango — with Texcoco being the largest and saltiest. The lakes were fed by rainfall and meltwater from the surrounding peaks and lost water only by evaporation, which made Texcoco progressively saltier as it concentrated dissolved minerals over millennia.
Tenochtitlan
The Mexica (commonly called Aztecs) founded their capital around 1325 on a small natural island near the western shore of Lake Texcoco. The choice was guided by religious prophecy — they were said to have been instructed to settle where they saw an eagle perched on a cactus eating a snake, the image still on the Mexican flag — but the location had practical advantages. The lake provided a natural defensive moat, freshwater could be brought from springs on the western shore via aqueducts, the wetland soil supported chinampas (raised agricultural beds built up from the lake bottom), and fish and waterfowl from the lakes provided a substantial portion of the diet. By 1519 Tenochtitlan had grown to between 200,000 and 400,000 inhabitants — making it larger than any contemporary European city — and the city had spread by reclamation and bridge-building to cover most of the western half of Lake Texcoco. Three causeways connected the island to the mainland, with drawbridge sections that could be raised in case of attack.
The Spanish Drainage Project
Hernán Cortés captured and largely destroyed Tenochtitlan in August 1521 after a 75-day siege in which the Spanish and their indigenous allies cut the city's causeways and aqueducts. The Spanish then chose to rebuild on the same site rather than relocate the capital to firm ground on the lake's edge, both for symbolic continuity with the Aztec capital and because the central square (today's Zócalo) was already laid out. The decision committed them to a 400-year struggle against the lake. Catastrophic flooding hit the new colonial city in 1555, 1604, 1607, 1629, 1707, 1714, and most disastrously from 1629 to 1635 — a period in which Mexico City was substantially underwater for several years and the Spanish briefly considered abandoning the site. Each flood pushed forward the engineering project to drain the basin.
The first major drainage tunnel — the Desagüe — was begun in 1607 under the direction of cosmographer Enrico Martínez, who started a 6.6-kilometre underground channel through the surrounding hills to give the basin an artificial outlet. The tunnel collapsed repeatedly and took decades to complete. Subsequent regimes — colonial, independent Mexican, Porfirian, revolutionary, and modern PRI-era — each added new drainage infrastructure. By the 1990s the lake was almost entirely gone except for a remnant section preserved at Xochimilco for cultural and ecological reasons.
The Sinking
The drained lakebed is composed of fine clay sediments accumulated over thousands of years. Clay sediments are compressible: when water is withdrawn from them, they consolidate and the surface drops. Mexico City's rapid 20th-century growth dramatically increased groundwater extraction from the aquifer beneath the city, which has caused the surface to subside at varying rates across the basin. The most extreme rates — 30 to 40 centimetres per year — are found in the western and southern districts that sit on the deepest clay layers. Several historic buildings have sunk over a metre below their original ground level, including the National Cathedral and the Palace of Fine Arts. The differential subsidence (some buildings sink faster than others) has produced visible tilting and cracking; the cathedral was undergoing constant repair as recently as the 2020s to keep its bell towers and the original Aztec foundations beneath it from drifting apart.
The Modern Megalopolis
The Mexico City metropolitan area today is the largest city in North America by population, with about 22 million inhabitants if you include the surrounding State of Mexico municipalities that form a contiguous urban area with the Federal District proper. The city is one of the most polluted in the Americas, partly because of the closed basin: pollution from cars, factories, and natural sources accumulates in the air and is held by the surrounding mountains. Inversion layers in winter trap smog. The 1985 earthquake, registering magnitude 8.0 on the Richter scale, killed at least 5,000 people (some estimates run to 10,000) and damaged thousands of buildings, again exposing the unusual amplification effects of seismic waves in the clay-rich former lakebed.
The Water Problem
The cruellest irony of Mexico City's geography is that the city that drained its lake now suffers from severe water shortages. The aquifer beneath the city is being depleted faster than rainfall can recharge it, since most of the basin surface has been paved over. Roughly 30 percent of the city's water is now pumped over the mountains from external sources — primarily the Cutzamala System, which lifts water 1,000 metres in elevation from the Cutzamala River to feed Mexico City. The system is expensive, energy-intensive, and increasingly stretched. Drought years like 2024 caused parts of the city to run out of piped water entirely for weeks at a time. Long-term planning increasingly recognises that the basin's closed hydrology will eventually force either dramatic reductions in population, large-scale water recycling, or a return to managed wetlands across parts of the basin that the city has historically suppressed.
Xochimilco and the Floating Gardens
The remnant of the lake system that survives today is at Xochimilco in the southern part of the city. The original chinampas, the agricultural beds built up from the lake bottom by the pre-Hispanic Mexica, are still being farmed by descendants of the Aztec lineage, who grow flowers, herbs, and vegetables on raised plots intersected by narrow canals. The colourful trajineras — flat-bottomed boats decorated with flower arches and named after women — carry tourists through the canals on weekends, accompanied by mariachi bands hired by the boat. UNESCO declared Xochimilco a World Heritage Site in 1987, and substantial conservation work is keeping the canals navigable despite the encroaching urban pressure. The area also harbours the axolotl, an endangered native salamander that has become both a symbol of the city and a recurring popular-culture reference.
What Comes Next
Mexico City's engineering future is dominated by water management. New deep drainage tunnels (most recently the Túnel Emisor Oriente, completed in 2019) carry sewage and stormwater out of the basin. Constructed wetlands and rainwater harvesting projects are being expanded. The Conagua water agency is working on aquifer recharge schemes that pump treated wastewater back into the ground to slow subsidence. None of these measures has yet stabilised the basin's long-term water balance. The city's habit of solving its current problems while postponing its underlying ones — established when the Spanish chose to rebuild Tenochtitlan rather than relocate the capital — continues. The 22 million people who now live in the basin are betting that the same approach will continue to work, at least for another generation or two.