Iowa' sunken highway, or the Atlantis Highway.

This highway was originally IA 40 (the Red Ball Route), then US 161. In 1938, 161 became US 218. 218 was re-routed in 1956, and abandoned when the Coralville dam was constructed. As a result, this once busy highway spends much of its time under water. Next time you are heading north on I-380, crossing the Iowa river, look to your right, and if the water is down, the highway will be visible.

http://iowahighways.home.mchsi.com/highways/us218.html