If ever there was a setting for a mystery, House Island would be it. It's the stuff that fuels childhood fantasies. Thought to be the site of the earliest European house built in Casco Bay, it was referred to as "the island with the old house on it," or House Island. But the island has seen more than just a house. In 1808, the US government built sprawling Fort Scammel to guard the main entrance to Portland Harbor. The fort was expanded during the Civil War. Later, the federal government constructed a quarantine station for immigrants on the other side of the island. Private tours of the historic facilities are available. Visitors can also enjoy island lobster bakes sea-side or in a heated club house.