"The Thousand Islands" is the name of a 50-mile chain of more than 1500 islands that straddle the U.S.-Canada border in the Saint Lawrence River as it emerges from the northeast corner of Lake Ontario. Some belong to the United States, others to Canada; the Canadian islands are in the province of Ontario (nearest town , while the the U.S. islands are in the state of New York. The islands range in size from 40 square miles to tiny uninhabited rock outcroppings (the criteria for "island" was that it must be above water level for 365 days per year, have a surface area larger than one square foot, and support at least one tree or shrub).

The area was a favorite summer vacation area for wealthy Americans and Canadians in the late 18th and early 19th century. Activities of the area include boating, fishing, birdwatching, camping, and diving. Points of interest include Boldt Castle on Heart Island, Singer Castle on Dark Island, popular fishing spot Eel Bay, and Deer Island, a retreat of Harvard's Skull and Bones Society.