The Aztecs constructed the Chinampas for agricultural cultivation, piling up mud from the lake's bottom, rich in organic matter and nutrients, and enclosing it with a fence of wooden poles. The depth of the ponds could be as much as 12 feet (3.6 m). They were designed by the city's engineers to provide crops year-round, even during periods of drought. The presence of chinampas in the city center is evidence that the urban environment was not considered dangerous at that time. It shows that the municipal government had regulations to protect citizens' safety. Chinampas were an important part of the city's economy because they provided food when other sources ran out or were not enough. They also provided a source of fertilizer - the plants and weeds that grew in them gave off excess nitrogen which helped make more corn grow better. Finally, they allowed people to escape severe storms or floods without having to leave the city. After the Spaniards arrived, the chinampa system was used for farming until about 1790. After that, it was abandoned because there was no longer need for an elaborate system to raise corn. Instead, the Europeans brought in horses and oxen to work the fields. This method was less labor-intensive but also less effective because it required a lot of land to keep up with the growing demand for corn.