Final answer:
The Serpent Mound was built by Native Americans, most likely by the Fort Ancient Culture around 1070 CE. It is an atypical mound at 1,300 feet in length as it lacks artifacts and burials, which are common features in other mounds of the region.
Step-by-step explanation:
The Serpent Mound was built by Native Americans. While the specific cultural group responsible for its construction has been debated, the prevailing theory ascribes it to the Fort Ancient Culture, with its creation dated to around 1070 CE. This mound-building society lived in the Ohio Valley and was significantly influenced by the contemporary Mississippian culture, whose urban center was at Cahokia in Illinois. Consideration has also been given to the possibility that the Fort Ancient Culture reworked or refurbished an earlier mound made by the Adena or Hopewell Cultures, known for their mound-building throughout the region.
The Great Serpent Mound is the largest serpent effigy in the world at approximately 1,300 feet in length. Unlike many other mounds of the Adena and Hopewell Cultures, this mound does not contain any artifacts or serve a burial purpose, making its attribution to a specific group and time more complex. Even so, the mound stands as an incredible testament to the sophisticated and settled peoples who cultivated maize, beans, and squash and established a stratified society with organized labor forces in North America long before European contact.