The Thousand Islands form an archipelago of 1,864 islands in the Saint Lawrence River between Ontario and New York, where the river leaves Lake Ontario. Shaped largely by the Frontenac Arch, the region combines striking scenery with hazardous navigation, clear water, and rich ecological value recognized by UNESCO in 2002. Known long before European settlement to the Iroquois Confederacy and Ojibwa peoples as Manitouana, it later became a frontier of war and trade, including action in the War of 1812. In the late 19th and early 20th centuries, it became a celebrated resort landscape, and today remains known for boating, fishing, diving, parks, and historic landmarks such as Boldt and Singer castles.