Hanging Rock and Wabash Reef National Natural Landmark is a small protected site on the south bank of the Wabash River near Lagro, Indiana, preserving a fossilized coral reef from the Silurian Period about 400 million years ago. Covering about 4 acres, it features a limestone reef deposit that rises about 75 feet above the river. Erosion by the Wabash undercuts the rock face, creating the overhang that gives Hanging Rock its name. Designated a National Natural Landmark in May 1986, the site is owned by Acres Land Trust and is open to the public from sunrise to sunset, with restrictions in place to protect it.