A 60-year-old woman suffered burns to her lower leg after she walked off trail and fell through a thin crust of ground into “scalding water” at Yellowstone National Park on Monday afternoon, the National Park Service said.

The woman, who is from Windsor, N.H., was walking with her husband and her leashed dog near the Mallard Lake Trailhead near the Old Faithful geyser when she broke through ground, according to a news release from the National Park Service. She suffered second- and third-degree burns on her lower leg and was transported by helicopter to Eastern Idaho Regional Medical Center to be treated. Park officials did not release the woman’s name, and said that the episode was under investigation.

The N.P.S. warned visitors to stay on trails and boardwalks in hydrothermal areas because of the fragile ground and the dangerously hot water below the surface. Pets are allowed in some areas of the park on a leash or in a crate, but they are not allowed in thermal areas, on boardwalks, on trails or in the backcountry in Yellowstone National Park.

Park officials said that this week’s accident is the first known thermal injury this year.

Yellowstone National Park, which is spread across parts of Wyoming, Montana and Idaho, has more than 900 miles of hiking trails and 15 miles of boardwalks around the park’s geologic and hydrothermal features, according to the park.

Since the park’s beginnings in 1872, 22 people have died from burns suffered after they entered or fell into hot springs there, which can reach temperatures hotter than 185 degrees Fahrenheit, according to United States Geological Survey. More visitors have been killed and injured by hot springs than by any other natural feature at the park, including wildlife, according to the N.P.S.

In June, a tourist at Yellowstone who ignored signs and crossed a fence to get near Steamboat Geyser in April was sentenced to a week in jail after being found guilty of “thermal trespass” and was banned from the park for two years.