Old South Boundary Road is a hiking, biking, and horse trail in Ontonagon County, Michigan. It is within Porcupine Mountains Wilderness State Park. It is 0.6 miles long and begins at 888 feet altitude. Traveling the entire trail is 1.3 miles with a total elevation gain of 175 feet. The Union Mine (elevation 889 feet) quarry is near the trailhead. There is also a parking.