The Great Serpent Mound in Ohio is a three foot high mound in the shape of a snake. I admit that a three foot high mound is not that impressive. However, it is a 1,300 feet long mound. Yup, that's more than four football fields. And apparently about 2,400 years old.
I was fascinated by this when I was younger. In high school, I used it as the basis for the first real role-playing game that I ever ran. The players were so afraid of it they ran back to town and expressed an interest in hunting and trapping for furs until they went up a level. This was not the last time I scared off players with structures that seemed pretty innocuous to me. I'm not sure why my idea of architecture is so frightening to other people.