One cave in Minnesota offers visitors an out-of-this-world experience, and the hike is only a little over a mile long.
To get to the cave, start at the St. Croix Boom Site, a Minnesota State Wayside and part of St. Croix National Scenic Riverway unit of the National Park Service. Only In Your State will take you the rest of the way:
"From the parking lot at the wayside, take the steps down to the river bank. From the base of the stairs, head south (i.e., turn to your right) and follow the trail along the base of the bluff for a short distance until you reach a small, sandy beach. In the limestone at the back of the beach, you’ll find a small cave. From the cave, turn back the way you came, past the stairs, for about half a mile. The narrow trail will skirt the bluff. When you reach a small creek, follow the creek upstream a short distance. Your efforts will be rewarded with a gorgeous little waterfall tumbling over the rock-lined river bank."
Keep in mind that when the river is high, the trail and the cave may be under water. In addition, The cave may be small, but it is very deep. It's not always treated with respect, and you may find graffiti on its walls.