The St. Croix Boom Site is a historic landmark that marks the place where logs were sorted and stored for the lumber industry from 1856 to 1914. Near the site, you can explore a cave that was once used by loggers and rivermen as a shelter and a storage place. The cave has a wooden door and a sign that reads "Boom Site Cave". Inside, you can see graffiti from past visitors and feel the cool air. The cave is a hidden gem for adventurous hikers who want to experience a bit of history and mystery.