What is the largest settlement of the mound builders?

"The best known flat-topped pyramidal structure, which is also the largest pre-Columbian earthwork north of Mexico at over 100 feet (30 m) tall, is Monk's Mound in Illinois at Cahokia."The mound builders were originally believed to be extinct long before the Spanish arrived, but disease probably spread ahead of the Spanish and wiped them out before we ever visited their part of the US.
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