The land was first leased for mineral exploration in 1892, part of the frenzy that followed the discovery of iron ore across the Mesabi Range in northeastern Minnesota. A second lease came in 1900, and active mining began in 1913. Over its operational life, Hill-Annex produced 63 million tons of iron ore, making it the sixth largest producer in the state. The open pit deepened year by year, carving through ancient rock while generations of miners worked the benches above. Around the rim, the infrastructure of extraction took shape: an office building and laboratory around 1930, a maintenance shop from the same era, a truck repair shop and wooden water tower dating to 1919, and a community club built in 1915 where miners and their families gathered. The former mine was listed on the National Register of Historic Places in 1986 as a historic district.

Mining stripped away more than iron ore. It exposed the Coleraine Formation, a Cretaceous-era geological deposit found only in Minnesota. In 2014, the Hill Annex Paleontology Project began as a volunteer-led study through the Science Museum of Minnesota, later moving to the Minnesota Discovery Center in Chisholm. The results were remarkable. In 2015, researchers found the end piece of a toe bone from a dromaeosaur -- a raptor roughly five to six feet tall that lived during the Late Cretaceous Period. It was only the second piece of dinosaur remains ever found in the state of Minnesota. The pit also yielded ammonites, crocodile remains, and various species of fish, sharks, turtles, crabs, mollusks, snails, and clams. An industrial wound in the landscape had become a window into a time when this part of Minnesota lay beneath a shallow sea, teeming with life that predated the iron ore by tens of millions of years.

After mining ceased and the site became a park, conservationists began adding vegetation to rehabilitate the scarred landscape. The effort paid off. Grouse, deer, coyotes, timber wolves, and black bears took up residence in the recovering terrain. Raptors returned too -- not the Cretaceous kind, but living ones: eagles, peregrine falcons, and hawks circling above the old pit. The juxtaposition was striking: a place gutted by industrial extraction becoming habitat for some of Minnesota's most iconic wildlife. The Iron Range is full of such transformations, where abandoned pits fill with water to become swimming holes and forests slowly reclaim the spoil piles. Hill-Annex was the version of that story with a state park sign out front.

The Minnesota Department of Natural Resources took over the site in 1991, but the arrangement always carried an asterisk. When MNDNR assumed responsibility, they noted the transfer was "consistent with the historical expectation that mining would return to this site." In 2024, that expectation became reality. The DNR permanently closed Hill-Annex Mine State Park, clearing the way for iron ore extraction to resume. The decision reflected a tension woven through the entire Mesabi Range: the same geology that makes these places historically fascinating also makes them economically valuable. Hill-Annex served as both museum and monument for decades, its historic buildings and fossil beds drawing visitors into the story of Minnesota's iron heritage. Now the pit will deepen again, the trucks will roll, and the fossils still embedded in the Coleraine Formation will either be carefully salvaged or crushed into aggregate.