> the chinampas were started as floating mats of reeds with enough soil on top to to grow crops. Every year you add a new layer of soil and fertilizer and the i

> the chinampas were started as floating mats of reeds with enough soil on top to to grow crops. Every year you add a new layer of soil and fertilizer and the island sinks a little more, until eventually it is touching the bottom of the lake several hundred years later. That description of chinampas is a popular misconception. They're "floating" in the sense that they're in the middle of a lake but there's no way to float any nontrivial amount of soil on top of mats of reeds, there just isn't enough buoyancy and a cubic meter of damp soil weighs over a metric ton. The reeds were used to create underwater fences that would keep the underwater pile of soil from spilling out into a giant mound, thus allowing the soil to be piled in a vertical pillar that easily reaches above the water. After the chinampa was filled, they planted trees along the corners and edges so that the roots would further hold the chinampa together. They added night soil dredged from the lake on top every year to replenish the nutrients and replace the soil that gets eroded away by the lake but they're filled to the bottom when they're built.