Set on a 2,500-acre bluff overlooking the west bank of the Mississippi River, Effigy Mounds National Monument contains the country's largest publicly held collection of prehistoric earthworks. Visible via a web of hiking trails (14mi round-trip), nearly 200 burial mounds lie within the park's borders. Archaeologists have dated the mounds as far back as 500 BC and as recently as AD 1300, but little is known about the prehistoric Late Woodlands culture that created them.