The Strange Floating Island Up North
Locals claim it's the only one in the world, but I can't verify that.
Lake Dubonnet (also known as Mud Lake) is just south of Traverse City near Interlochen off US 31. It was formed when they dammed up the Platte River.
A kayaker's paradise, the clear, pristine lake is undeveloped and under fished. It is said to contain a bevy of Northern Pike.
But the strange thing about Lake Dubonnet is its floating island. What appears to be a solid island of pine trees and marsh is actually floating freely in the lake, unattached to the bottom.
Not much is known about the island or how it came into existence. In fact, the only proof that it floats (besides stories from local who have claim to see it move during high winds) comes from this geo-cache web site about a cache on the island, that changes GPS coordinates. (spooky)
If this Island has decided to float around and the coordinates are no longer accurate, just log the new coordinates for the next hunter. For fun, just log the coordinates that you found the cache at anyways.
Geocaching, if you don't know is "an item, typically a container holding a number of other items, that has been hidden at a location whose coordinates have been posted on the Internet" according to Google.
There are a couple of other mentions of the floating island on a fishing web site, and this Traverse City Record Eagle article about kayaking.
Outside of that, only you and I know, so SHHHHHH!!!!!