Approx. 140 miles to the entrance. Yellowstone National Park website: https://www.nps.gov/yell/index.htm
May through September: No key required, open from 8 a.m. to 6 p.m.
October through April: Key required, can be picked up at State Bath House, Chamber of Commerce & Hot Springs County Museum. There is a drop box at Legend Rock Petroglyph Site.