The most famous Adena earthwork is Serpent Mound. It is 5 feet high, 20 feet across, and 1330 feet long. It is formed in the shape of a snake. Serpent Mound - Serpent Mound is located near Cincinnati, Ohio. It is shaped like a huge curving snake. It is more than 2,000 years old and is over 1,300 feet long. The mouth of the snake is open and is swallowing something. Some people think the serpent is swallowing an egg.