The ancestors of the American Indians of the Eastern Woodlands built Serpent Mound, but archaeologists disagree about when it was built. Some argue that the Adena culture built it around 2,300 years ago, the same culture that built the large conical burial mound at the east end of the parking lot. Others think the Fort Ancient culture built it around 900 years ago; that culture built the small burial mound near the west end of the parking lot. Serpent imagery became especially important during the Mississippian period, which was contemporary with the Fort Ancient culture in southern Ohio, supporting the idea that the Fort Ancient culture built Serpent Mound. Future research may reveal its true age, but for now its date remains uncertain.