In the fall of 1520, Hernán Cortés sat in a camp outside the walls of a city he had already captured once and from which he'd been driven out in disgrace. He was writing a letter to King Charles V—detailed, rapturous, and in places outright dishonest. He described a huge island city in the middle of a lake, its causeways, aqueducts, markets, temples. He marveled at the engineering, compared Tenochtitlan to Seville and Córdoba. But this wasn't a tourist's report. It was political insurance: a man who had acted without the governor of Cuba's authorization and violated direct orders was explaining to his monarch why everything he'd done was necessary. Cortés's letter is one of the most detailed documents about Tenochtitlan on the eve of catastrophe. But it needs to be read with a correction: the author wanted to look like a hero, not a bandit.
Tenochtitlan didn't just stand "on a lake"—it was integrated into the water system so tightly that drawing a boundary between the city and Lake Texcoco was impossible. Founded in the 1320s on a swampy islet, by the early 16th century the city had grown to approximately 13 square kilometers. Population is estimated between 150,000 and 250,000 people—the Aztecs didn't conduct exact censuses, and Spanish sources contradict each other. For comparison: in 1500, Paris had about 200,000 residents, London—50,000, Seville—45,000. Tenochtitlan wasn't just a large city of its time. It was one of the largest.
Four main causeway-roads crossed the city: north (to Tlatelolco and Tepeyac), south (to Coyoacán), west (to Tlacopan), and east (to Iztapalapa). These causeways weren't just embankments—they included removable wooden bridges that could be dismantled in minutes, turning the city into a fortress. Inside Tenochtitlan there were almost no streets in the European sense: neighborhoods were connected by a network of canals plied by cargo-laden canoes. Bernal Díaz del Castillo, a soldier of Cortés, later recalled that the Spaniards first saw the city from the height of the Iztapalapa causeway and didn't believe their eyes: white temples, straight canal lines, multitudes of boats. Some decided it was witchcraft. Others compared it to illustrations from chivalric romances.
Fresh water reached the city through a double aqueduct from the Chapultepec springs, located several kilometers to the west. The aqueduct ran along the causeway, with one line operating while the second was cleaned or repaired. Water flowed through ceramic pipes and was distributed through a system of fountains and reservoirs. But the main engineering challenge wasn't water delivery—it was protection from it: Lake Texcoco was brackish, and rains and rivers from the western mountains regularly raised its level. In 1449, still under the reign of Moctezuma I, the city nearly flooded. After that, the Nezahualcoyotl dike was built—a stone wall approximately 16 kilometers long that separated the eastern, salty part of the lake from the western, fresh part, and protected Tenochtitlan from storm surges. The city existed in constant equilibrium: too much water—flooding, too little—a drinking water crisis.
Chinampas aren't "floating islands," as Spaniards often called them and tourist guides continue to call them. They're stationary artificial fields created in shallow water. The technology worked like this: wooden stakes were driven into the lake's muddy bottom, enclosing rectangular plots about 30 meters wide and up to 100 meters long. Layers of algae, bottom silt, and soil were piled inside until the plot rose above water level. Edges were reinforced with willow trees whose roots bound the soil together. Narrow canals for canoes were left between chinampas. Productivity was phenomenal: up to seven harvests per year on some crops. Chinampas fed a city that couldn't sustain itself through trade or tribute alone. This wasn't just agricultural technology—it was the foundation of Tenochtitlan's metabolism.
Cortés devoted several pages of his letter to describing the Tlatelolco market—Tenochtitlan's northern quarter, which had once been a separate city. By his estimate, up to 60,000 people gathered here daily, and on the big market day, which came every five days—even more. They sold everything: corn, beans, cacao, jade jewelry, jaguar pelts, quetzal feathers, slaves, dogs for meat, ceramics, mats, tattoo dyes. There was a system of weights and measures; judges settled disputes on the spot. Cortés compared it to Salamanca. But the market wasn't just a trading floor. It was a logistics hub: tribute goods paid to Tenochtitlan by hundreds of subordinate cities were redistributed here among the elite, priests, warriors, artisans. The market was the visible part of the empire—the part that worked every day, not just on sacrifice days.
The city's center was the Templo Mayor—a double pyramid about 60 meters high, topped by two temples: one dedicated to Huitzilopochtli, god of war and sun, the second to Tlaloc, god of rain. This duality wasn't accidental: Tenochtitlan needed both military victories, which brought tribute and captives, and rain, which fed the chinampas. The temple complex included dozens of buildings: schools for priests, ball courts for the ritual game, skull repositories of sacrifice victims (tzompantli) stacked into neat walls. The Spaniards saw these walls and felt revulsion. But for the Aztecs, sacrifice wasn't sadism—it was cosmic necessity: the sun needed blood to rise each morning. The world was fragile, and the gods demanded constant feeding.
Next to the Templo Mayor stood the palaces of Moctezuma II—Tenochtitlan's ruler from 1502 to 1520. The complex included living quarters, reception halls, a menagerie with jaguars and eagles, aviaries with hundreds of bird species, a botanical garden. Bernal Díaz recalled that there were about 300 people caring for the birds and animals alone. Moctezuma received ambassadors from different parts of the empire, listened to reports about borders, taxes, wars. The Spaniards, entering the palace for the first time in 1519, couldn't grasp the scale: they thought they were dealing with an Eastern-type despot, like a Turkish sultan. In reality, Moctezuma ruled through a complex system of councils, military orders, and priestly colleges. He wasn't an absolute monarch but first among the elite. But this is precisely what made him vulnerable: when the Spaniards took him hostage in November 1519, the empire's governance system began to malfunction.
The city was divided into four large quarters (Nahuatl: campan), each including dozens of smaller neighborhood-calpulli. Calpulli were the foundation of social structure: communities with their own temples, schools, warehouses, leaders. Taxes were collected at the calpulli level; military levies came from there too. But Tenochtitlan also had another organizational axis—not horizontal (by quarters) but vertical (by status). The elite—pipiltin—lived in stone houses, wore gold and feather ornaments, ate meat and cacao. Commoners—macehualtin—built from adobe, wore uncolored cotton clothing, ate corn and beans. Slaves, war captives, foreign merchants. The city wasn't a monolith but a set of layers that mixed in the marketplace and dispersed at night.
June 30, 1520—the night the Spaniards called Noche Triste, "Sad Night." The Spaniards had been in Tenochtitlan for half a year as guest-hostages. Cortés had gone to the coast to defeat a punitive expedition from Cuba. In his absence, Pedro de Alvarado, left in command, ordered a massacre during the festival of Toxcatl. The exact reason is unclear: Alvarado claimed an uprising was being prepared, but most likely the Spaniards simply panicked at the gathering of armed Aztecs. They killed several hundred nobles and priests right in the sacred courtyard of the Templo Mayor. The city exploded. The Aztecs blockaded the Spaniards in the palace of Axayacatl, where they sat for several weeks until Cortés returned with reinforcements. But reinforcements didn't save the situation. Cortés tried to use Moctezuma to calm the crowd—Moctezuma came out onto a balcony and was pelted with stones by his own subjects. He died a few days later. The cause of death is unclear: the Spaniards claimed he died from wounds received from the stones, but there's a version that the Spaniards themselves killed him when he ceased to be useful.
On the night of July 1, 1520, the Spaniards attempted to break out of the city. The plan was: carry out a portable wooden bridge, move along the western causeway toward Tlacopan, throw the bridge across gaps in the causeway that the Aztecs had managed to dismantle. The plan failed at the first gap: the bridge got stuck under the weight of people and gold. The Aztecs attacked from canoes, from rooftops along the causeway, throwing Spaniards into the water. Armor dragged them to the bottom. Most of the dead drowned rather than being killed in combat. Cortés lost 600 to 800 Spaniards out of approximately 1,300, plus several thousand Tlaxcalan allies. Fewer than half the Spaniards escaped from Tenochtitlan, nearly weaponless and without the treasury. Cortés, according to legend, sat under a tree near Tlacopan and wept. But he didn't weep for long.
Cortés spent the following months on diplomacy and logistics. He assembled a new army—but its core wasn't Spaniards but allied Indians. The Tlaxcalans, enemies of Tenochtitlan since the 15th century, fielded up to 50,000 warriors. The Texcocans, who had once been partners of the Aztecs in the Triple Alliance, switched to the Spanish side after their ruler Cacamatzin was killed on the night of the Spanish flight from the city—either by the Aztecs or by the Spaniards themselves, who no longer needed the captive. Cholula, Huexotzinco, dozens of small cities—all wanted revenge. For them, Cortés wasn't a conqueror but a tool. The most crucial element of the new strategy was 13 brigantines—small sailing vessels that Cortés ordered built on the shore of Texcoco. Timber for them was cut in the mountains and dragged dozens of kilometers by the Tlaxcalans. The ships were launched in April 1521. From then on, Tenochtitlan was surrounded not only from the causeways but also from the water.
The siege began in late May 1521 and lasted almost three months—about eighty days. The Spaniards and their allies blocked all three main causeways. Brigantines patrolled the lake, cutting the city off from the chinampas and from external trade. Every day the Spaniards together with the Tlaxcalans stormed Tenochtitlan's quarters, capturing canal after canal, house after house, and filling the canals with stones and rubble so the Aztecs couldn't retake them at night. The Aztecs fought under the leadership of a new ruler—Cuauhtémoc, a young military commander elected after the death of Moctezuma and his successor Cuitláhuac, who died of smallpox. The Aztecs set ambushes, attacked from canoes, dropped stones from rooftops. In June 1521 they captured several dozen Spaniards and sacrificed them atop the Templo Mayor—the victims' heads and limbs were displayed for the besiegers to see, to break their spirit. But hunger did its work faster. The city ran out of water—the aqueduct had been cut. Food ran out. Dysentery began. Corpses weren't removed—no one to do it and no reason. By the end of the siege, people were eating bark, leather, grass. The Spaniards, entering captured quarters, tied rags over their faces to avoid suffocating from the smell of decomposition.
August 13, 1521, Cuauhtémoc attempted to escape by canoe across the lake. He was intercepted by one of the brigantines. He surrendered. A few days later, Cortés ordered the assault to cease—there was no one left to resist. Of 200,000 residents, by various estimates, 30,000 to 60,000 survived. Some fled before the siege began, some died of hunger and disease, some were killed in combat. Tenochtitlan ceased to exist as a city. What remained were ruins, canals choked with bodies, and smoke from fires burning the dead to stop epidemics.
Cortés made the decision to build a new capital on the site of the old one not out of sentimentality but logistics: Tenochtitlan was a hub of causeways, canals, roads. Its location ensured control over the valley. But building a city on a lake seemed absurd to the Spaniards. Their urban planning tradition demanded plazas, straight streets, stone buildings on dry land. And Cortés began remaking the landscape.
First they filled in Tenochtitlan's central canals, using stone from demolished temples and palaces. The Templo Mayor was dismantled almost to its foundation—blocks went into foundations for new cathedrals and residences. By 1524, construction began on Mexico City Cathedral on the site of the sacred district. Construction took 240 years—the cathedral wasn't finished until 1813, redesigned several times because the building constantly settled. The foundation rested on soft, water-saturated soil, and stones slowly sank downward. Spanish engineers didn't understand why Aztec temples had stood for centuries while their cathedral was cracking. The answer was simple: the Aztecs built on platforms of compacted rubble and wood that distributed weight. The Spaniards just piled stone on stone.
Canals were filled gradually—a process that stretched over 300 years. Some were used until the 18th century for transporting goods, but most were filled by 1700 and became streets. They began draining Lake Texcoco almost immediately: the Spaniards feared floods and considered water a source of disease. The first major project—the Huehuetoca tunnel, begun in 1607 after the catastrophic 1604 flood that submerged the city for several years. The 6.5-kilometer tunnel was supposed to divert water from the lake into the Tula River valley. Tunnel construction began in 1607 under German engineer Enrico Martínez—and dragged on for two centuries: the mountain excavation, converted into an open canal, was only completed by 1789 and cost thousands of Indian workers their lives—cave-ins, landslides, floods. The tunnel never worked perfectly, but it set the trend: the Spaniards wanted to remove water from the valley, not live with it.
By the 19th century, most of Lake Texcoco had disappeared. A network of swamps and small ponds remained, which completely dried up in the 20th century. Mexico City grew on the bottom of the former lake, on layers of silt, clay, and volcanic sand. The city built houses, laid roads, pumped water from underground. And began to sink.
Modern Mexico City rests on a layer of clay and silt 30 to 100 meters thick, beneath which lie volcanic rocks. This clay was saturated with water while the lake was here. But from the 1900s onward, Mexico City began pumping groundwater at an accelerating rate. The first wells appeared in 1847, but mass pumping began in the 20th century, when the population grew from 500,000 in 1900 to 21 million in 2020. The city consumes about 70 cubic meters of water per second, of which up to 70% comes from underground aquifers. Water disappears—clay compresses. The city subsides.
The subsidence rate is uneven: in central Mexico City it reached 30–40 centimeters per year in the 1950s, now slowed to 10–20 centimeters per year in some areas, but in the Iztapalapa district—the very one from which the causeway to Tenochtitlan ran—it still reaches up to 30 centimeters per year. From 1900 to 2020, the city center dropped 9–10 meters. This is visible in buildings: foundations of old colonial houses protrude above ground because the street around them has sunk. The cathedral is tilted—the north tower is several meters lower than the south. In the 1990s they tried to straighten it by pumping concrete under the foundation, but the process can't be stopped, only slowed.
Subsidence creates chain effects. Sewer pipes that should slope downward end up higher than their outlets. Wastewater doesn't drain by gravity; it has to be pumped out. The drainage system built in the 1970s is deformed—pipes cracked, mains bent. Rainwater doesn't sink into the ground because the city is covered with asphalt, and doesn't drain into sewers because they're overloaded. During heavy rains, districts flood for several hours, and water slowly evaporates because pumping can't keep up. A city built on the site of a lake, cut off from water by centuries of drainage, now drowns in rain.
The connection to the colonial past here isn't linear. You can't say Mexico City is subsiding because the Spaniards destroyed Tenochtitlan. Subsidence is caused by water pumping that began in the 20th century. But the Spaniards laid the groundwork: they filled the canals, drained the lake, eliminated the chinampa drainage system that allowed water to distribute. The Aztecs lived with water, the Spaniards tried to get rid of it, and modern Mexico City pays for that decision. The city isn't settling because of old temple stones but because of voids left by pumped water. But those voids appeared where there was once a lake that disappeared because the Spaniards decided the city should be dry. In a sense, Mexico City is subsiding under the weight of its own choice—but that choice was made 500 years ago, by people who didn't understand they were building a city on a cushion of water.