Established in 1872, Yellowstone National Park is a national treasure. Located primarily in Wyoming, the park is so massive that it extends into Montana and Idaho and has five separate entrances. Yellowstone is a real hothead: It contains half of the world's known geothermal features, including geysers, hot springs, and boiling mud pots. The most famous is Old Faithful, a geyser that has reliably erupted for decades. You'll need a vehicle to get the most out of the park, but its sprawling size doesn’t mean you’ll be in isolation. In fact, you'll be joined by more than a million visitors during the busy summer season. Prepare for traffic jams caused by humans and bison alike. Luckily, there are plenty of places to rest around the park, including nine lodges and 2,000 campsites.