Serpent Mound - http://www.ohiohistory.org/places/serpent/

Mound City Hopewell Culture National Park -  http://www.nps.gov/hocu/

Fort Ancient - http://www.ohiohistory.org/places/ftancien/

Seip Mound State Park - http://www.stateparks.com/seip_mound.html

Fort Hill State Park - http://www.stateparks.com/fort_hill.html

Flint Ridge State Memorial - http://www.culture.ohio.gov/project.asp?proj=flint

Other Ohio Earthworks - http://www.ibsgwatch.imagedjinn.com/learn/ohio.htm