Mud Volcano is one of Yellowstone National Park's most fascinating — and most pungent — hydrothermal areas, tucked along the Grand Loop Road in the Hayden Valley region between Yellowstone Lake and Canyon. Unlike the park's colorful hot springs, this is a moody, acidic landscape of bubbling mud pots, hissing fumaroles, and churning caldrons that gurgle and belch in a haze of sulfur, a vivid reminder of the restless volcano beneath your feet.

In 1870 Mud Volcano had a cone-shaped mud deposit that covered the feature; by 1872 a thermal explosion had blown that mud away to reveal what we see today. A boardwalk loop of roughly two-thirds of a mile (with some stairs and hills) tours the area's memorable features, including Dragon's Mouth Spring — a cavern that surges and roars like a beast — along with Churning Caldron, Black Dragon's Caldron, Sour Lake, and Sizzling Basin. This is one of the most acidic hydrothermal areas in the park, so the strong rotten-egg smell of hydrogen sulfide is part of the experience.

The area is easy to reach and free to visit as part of park admission, and it's a quick, rewarding stop between Yellowstone Lake and the Grand Canyon of the Yellowstone. Bison frequently graze and rest nearby — give them a very wide berth (at least 25 yards), as they are wild and dangerous.

Stay on the boardwalk at all times. The ground here is a thin, fragile crust over near-boiling, acidic water, and leaving the trail is both illegal and potentially fatal. The area is open year-round, though the road is closed to regular vehicles in winter (accessible by snowcoach or ski). Bring your camera and a tolerance for sulfur, and enjoy one of Yellowstone's wildest geological showcases.