Cahokia is not only the largest prehistoric metropolis north of Mexico, it also has the largest prehistoric mound. Monks mound measures at least 291 m (955 feet) north-south and 236 meters (774.3 feet) east-west. Its height is between 28 meters (92 feet) and 30 m (99 feet). Research has revealed new puzzles, suggesting that it is a very complicated structure. Undoubtedly, the builders did some clever engineering to keep this and other mounds from collapsing. This should not be a surprise, because even the 5000 – 6000 years ago megalithic tombs of the Funnel Beaker culture in Central and North Europe exhibit special internal features designed to drain water away. In all, there are at least 104 mounds at Cahokia.