Butte owes its existence to placer gold discovered on Silver Bow Creek in 1964, but the gold soon played out, and demand for Butte's copper made it "the richest hill on Earth." The hill produced over two billion dollars worth of gold, silver, copper and zinc, while drifts, winzes and stopes honeycombed the underground into a labyrinth that still extends beneath the city. There were over 3,000 miles of workings and shafts, some reaching a depth of 4,000 feet. In 1955, the Anaconda Company opened the Berkley Pit to mine low-grade metals, and the pit swallowed up Meaderville, McQueen, and copper king W.A. Clark's Columbia Gardens amusement park. Butte has a cosmopolitan population derived from the four corners of the world. It was a bold, unashamed, hell-roaring camp in days gone by and remains at the heart of American unionism, with a labor history second to none in the nation. The National Park Service recognized Butte as a National Historic Landmark in 1961, and in 2006 extended the landmark to recognize Butte, Walkerville, and Anaconda as a labor corridor whose intertwined histories encompass nearly 10,000 acres, the largest National Historic Landmark in the West.