Milton House - Credit: Wikimedia Commons
Milton House is a historic building located at 18 South Janesville Street in Milton, Wisconsin. It was a stop on the Underground Railroad, a network of people and places that facilitated the movements of escaped slaves. The building has been asserted to be the first grout building built in the United States; although that claim is dubious, the house's grout construction apparently was influential. The AIA Journal called it the "Oldest standing concrete structure of any consequence.""U.S. Architectural Landmarks in Concrete," AIA Journal, Vol. 50, No. 3, pg. 68, September 1968. It is also known for its hexagonal shape. Read more on Wikipedia
Source: en.wikipedia.org