Which of these features is the unique geological/geographic characteristic of Isle Royale in Lake Superior?

Siskiwit Lake is the largest lake on Isle Royale which is the largest island of Lake Superior. Siskiwit Lake itself contains several islands, including Ryan Island, the largest, which contains Moose Flats, a seasonal pond, which contains Moose Boulder, a very particular rock. When Moose Flats is a pond, Moose Boulder becomes the largest island in the largest lake on the largest island in the largest lake on the largest island in the largest freshwater lake in the world - a particular and unique sequence of island configurations.

Isle Royale is part of the U.S. state of Michigan. It is 45 miles (72 km) long and 9 miles (14 km) wide, with an area of 206.73 square miles (535.43 square km), making it the fourth-largest lake island in the world. The highest point on the island is Mount Desor at 1,394 feet (425 m), or about 800 feet (240 m) above lake level.

There are seasonal passenger ferry services to Isle Royale from Grand Portage, Minnesota; Copper Harbor, Michigan; and Houghton, Michigan. There is also a seasonal sea plane service. There are no roads on the island, and wheeled vehicles or devices, other than wheelchairs, are not permitted. Rock Harbor has wheeled carts available to move personal belongings from the Rock Harbor marina to the cabins and hotel. Also the National Park Service employs tractors and a few World War II jeeps to move items around the ranger station area at Windigo, Rock Harbor, and Mott Island.