For the first time in history, Deep Tunnel, Chicago's massive system of underground tunnels and reservoirs designed to hold flood water, is almost completely full after a weekend of heavy rain. Built over more than a half century, Deep Tunnel sends floodwaters from Chicago and nearby suburbs through more than 100 miles of tunnels into three reservoirs located just outside of the city. It was designed to help protect the river as well as Lake Michigan. The Thornton reservoir in South Holland was