Maintained within a park by Ohio History Connection, it has been designated a National Historic Landmark by the United States Department of Interior. The Serpent Mound of Ohio was first reported from surveys by Ephraim Squire and Edwin Davis in their historic volume Ancient Monuments of the Mississippi Valley, published in 1848 by the newly founded Smithsonian Museum.
Researchers have at different times attributed construction of the mound to two different prehistoric indigenous cultures. Originally thought to be Adena in origin, a 1996 carbon dating study led scholars to believe the mound was built by members of the Fort Ancient culture around 1070 CE. More recent carbon dating done in 2014 places the mound's construction at around 300 BCE, once again suggesting Adena construction. Serpent Mound is the largest serpent effigy in the world.
Including all three parts, the Serpent Mound extends about 1,376 feet (419 m), varies in height from less than a foot to more than three feet (30–100 cm), and has a width of 20 to 25 feet. Conforming to the curve of the land on which it rests, with its head approaching a cliff above a stream, the serpent winds back and forth for more than eight hundred feet and seven coils, and ends in a triple-coiled tail.
The shape itself consisted mostly of a layer of yellowish clay and ash that was reinforced with a layer of rocks, and then covered with a layer of soil. The serpent head has an open mouth extending around the east end of a 120-foot (37 m)-long hollow oval feature that may represent the snake eating an egg, though some scholars posit that the oval feature symbolizes the sun, the body of a frog, or merely the remnant of a platform. The effigy's extreme western feature is a triangular mound approximately 31.6 feet (9.6 m) at its base and long axis. There are serpent effigies in Ontario and Scotland that are very similar.
According to wikipedia