For the first time in five years, Lake Superior, the world's largest freshwater lake, has frozen enough to allow visitors from northern Wisconsin to walk across the lake to the ice caves of the Apostle Islands National Lakeshore. According to Reuters, officials declared the trek "low risk" about a month ago, and nearly 35,000 people have made the mile-long hike since then. According to the NOAA, the Great Lakes have reached their broadest ice coverage in 20 years, at 88 percent -- with Lake Superior at about 95 percent. Over the weekend, Reuters photographer Eric Miller trekked out on the ice, capturing these images. Update: Photographer Paul Johnson was kind enough to share a few of his nighttime ice cave photos as well, numbers 16-18 below.