Portsmouth, Ohio, is on the Ohio river about 90 miles south of Columbus. I've visited here a few times and highly recommend the site (as well as the other sites in Ohio).
Portsmouth is the location of a complex of earthworks dating from 100 BC to 500 AD, built by Adena and Hopewell cultures. Across the river in Kentucky is an unusual earthwork that was plowed up for cattle grazing.
|Squier and Davis survey (click to enlarge)|
There's a nice overview with references on wikipedia.
This site has some additional info:
Greg Little tweeted about it recently: