Mud Volcano

Mud Volcano is a bubbling mudpot at the heart of Yellowstone’s Mud Volcano thermal area. A short boardwalk-and-paved loop links it with Mud Caldron, Mud Geyser and Dragon’s Mouth Spring, so you get several different looks at the park’s restless ground in one stop. Expect gray mud, steam and a rotten-egg sulfur smell rather than bright blue pools; that smell can be strong enough to put some people off.

The full route is about 0.6 mile, with boardwalk and pavement, and takes about one to two hours with stops. Go clockwise for the gentler direction; the longer hillside loop has a couple of steep sections and stairs. There’s a shorter lower loop around Mud Volcano and Dragon’s Mouth. It meets accessibility guidelines, though stairs lead down to Dragon’s Mouth’s viewing platform. Parking and an accessible restroom are at the trailhead, about six miles north of Fishing Bridge. Pets aren’t allowed on the trail.

In winter, access follows the park road and may be by snowcoach; expect snow and ice on the trail. Bison and calves frequent the area and sometimes use the walkways, so wildlife can shape how much of the loop you can cover.

The mud is acidic because microorganisms use hydrogen sulfide rising from deep underground and help turn it into sulfuric acid. That acid breaks rock down into wet clay; other gases escaping through the clay make it bubble. The mud can be thinner or thicker as water supply changes with precipitation and the seasons. Hydrogen sulfide is also what gives the basin its rotten-egg smell.

Dragon’s Mouth Spring is a separate feature along the same loop, not the Mud Volcano vent. Its cave-like opening releases steam and pressure bubbles that boom and gurgle against the roof. The main mudpot is a close, textural photo: frame the pale crater wall with its gray bubbling pockets. Dragon’s Mouth gives you a darker opening with steam for a very different scene.

The feature once formed a cone about 30 feet high and wide. A likely violent eruption blew out its side, leaving the crater seen today. When Nathaniel P. Langford visited in 1870, he saw a seething mass of bubbling mud. The water continues to undercut the crater’s back wall, which is rich in iron sulfides.

The hills east of the trail are part of Sour Creek Dome, which shifts as the magma chamber below changes. Faults converge here and earthquakes are common, so scientists monitor the dome. The same walk is good for wildlife: look for marmots, Clark’s nutcrackers, mule deer and elk, while bison often wander near or onto the boardwalk. Grizzly bears feed on winter-killed animals here in spring.