This small cave was believed to have been used by Native Americans as a burial site. During the height of the fur trade in North America it was also a place of refuge. Fur trader Alexander Henry wrote about it in "Memoirs," recounting his discovery of bones littered about the cave. It's a popular tourist site as well as one of the stops on guided tours of the island. You can sign up for tours of Mackinac some of which will include the cost of the ferry in the pricing.