Michio Kaku Warns A 100ft Wide Crack JUST Opened The Yellowstone Volcano! The oldest national park in the world, Yellowstone is not merely a treasure of the United States. Yellowstone, which opened its doors in 1872, spans 3,472 square miles (8,987 square kilometers) across three states. Nearly 3 million people visit the park each year to take in the breathtaking natural scenery, which features the Old Faithful geyser as well as numerous hiking trails, mountain peaks, and hot springs.