Picture a city more crowded than most cities in Europe at the time — sitting right in the middle of a lake. Not beside it. In it. The Aztecs called it Tenochtitlan, and it shimmered like something out of a dream. So how do you build a great city on water? You start, oddly enough, by being told to.

The story the Aztecs told goes like this: their god promised they would find their home where they saw an eagle perched on a cactus, eating a snake. After years of wandering, they spotted exactly that — on a soggy little island in the middle of a lake. Not the comfiest real estate. But a promise is a promise.

Now, the lake — Lake Texcoco — was not their first choice for a reason. It was shallow, muddy, and in some places too salty to drink. Other peoples already owned the nice dry land all around it. The island was leftover scraps nobody wanted. Which turned out to be the Aztecs' secret superpower: the water that made it hard to build also made it hard to attack.

To grow food, they invented floating gardens called chinampas. Here's the trick: they staked out a rectangle in the shallow water, wove a fence of branches around it, then piled in mud scooped from the lake bottom until it rose above the surface. They planted willow trees at the corners, whose roots reached down and anchored the whole plot in place.

These chinampas were ridiculously good farmland. The lake kept the soil damp from below, so crops grew thick and fast — corn, beans, squash, chili, flowers — sometimes several harvests a year. Hundreds of these green rectangles spread across the water, sliced apart by narrow canals. Need to visit your garden? You paddled there.

A city in a lake has one obvious problem: how do you leave? The Aztecs built long raised roads called causeways — earth and stone packed into solid bridges — stretching from the island to the shore. They left gaps in them crossed by wooden bridges that could be lifted away. Roll up the bridge, and the whole city becomes a fortress with a moat.

Fresh water was the next puzzle, since lake water was too salty to drink. So they built an aqueduct — a long stone channel running along a causeway — carrying clean spring water all the way from a hill called Chapultepec into the heart of the city. It even had two pipes side by side, so one could be cleaned while the other kept the water flowing.

With food, roads, and water solved, the city bloomed. Stone temples climbed toward the sky. Markets buzzed with thousands of traders selling everything from cocoa beans to feathers. Streets were sometimes water and sometimes stone, so you might walk to a friend's house — or paddle. At its peak, around 200,000 people lived here, making it one of the largest cities on Earth.

So that's the answer: the Aztecs didn't fight the lake — they made friends with it. They turned mud into gardens, water into highways, and a swampy scrap nobody wanted into a glittering capital. Engineering, patience, and a stubborn promise about an eagle. Not bad for the worst real estate on the lake.

And that eagle on the cactus, the one that started it all? The Mexican people never forgot it. Today, that very same eagle — perched on a cactus, snake in its beak — sits right in the center of Mexico's flag. A whole nation, still pointing back to one stubborn little island in a lake.