Due to its unique geography, being made of two peninsulas surrounded by the Great Lakes, Michigan has depended on many ferries for connections to transport people, vehicles and trade. The most famous modern ferries are those which carry people and goods across the Straits of Mackinac to the car-free Mackinac Island but before the Mackinac Bridge was built, large numbers of ferries carried people and cars between the two peninsulas. Other ferries continue to provide transportation to small islands and across the Detroit River to Canada. Ferries once provided transport to island parks for city dwellers. The state's only national park, Isle Royale cannot be reached by road and is normally accessed by ferry. The largest ferries in Michigan are the car ferries which cross Lake Michigan to Wisco