Steamboat’s toxic cave is home to weird worms, crystal-forming bacteria and now, possibly, a National Natural Landmark |

Steamboat’s toxic cave is home to weird worms, crystal-forming bacteria and now, possibly, a National Natural Landmark

Sulphur Cave on Howelsen Hill is up for the designation

Snottites hang as gypsum crystals form on the walls of the Sulphur Cave.
Courtesy city of Steamboat Springs

Editor’s note: Entry into the Sulphur Cave is prohibited and can result in death or harm to your health. Members of the public should not enter the cave.

STEAMBOAT SPRINGS — The air inside is deadly. It has one-of-a-kind worms that survive in a mix of elements that is toxic to humans. Gooey bacteria drip from the ceiling, as crystals of gypsum are slowly forming, and it’s right under our feet. 

The Sulphur Cave at Howelsen Hill is under consideration for a National Natural Landmark through the National Park Service. At Tuesday’s Steamboat Springs City Council meeting, city staff will seek direction from City Council as to whether they should pursue the designation for the unique cave.

There are a multitude of features that make the cave distinct.

It’s likely the first cave documented in Colorado, described in 1843 by Thomas Jefferson Farnham in Travels in the Great Western Prairies.

Then there’s the atmosphere. Breathing in the mix of hydrogen sulfide, carbon dioxide and carbon monoxide in the cave can kill a person. Even one or two breaths could knock you out, Fred Luiszer, a University of Colorado scientist who specializes in caves told Steamboat Pilot & Today in 2008.

Dagny McKinley, the Routt County author who wrote The Springs of Steamboat: Healing Waters, Mysterious Caves and Sparkling Soda, said it’s rumored that the Ute executed tribe members who were condemned to death at the cave, though she added this is only rumor, as there is no documentation supporting the tale.

According to the story, six-armed warriors forced the condemned to enter the cave.

“They would be taken to the mouth of the cave and forced to walk into it, and if the person refused to walk into the cave at all, then they were shot by poisonous arrows, which were excruciatingly painful and cause death,” McKinley said. 

There are a few reports of people entering the 180-foot cave, using gas masks, and living to tell about it since then, but only for minutes at a time. In 1962, an Italian exchange student with caving experience entered the cave with an oxygen mask and still came out, convulsing and falling in and out of consciousness when it became faulty, according to an archived edition of Steamboat Pilot.

“Definitely stay out,” McKinley said. “It’s not a joke that it is toxic and will kill you. It’s not one of those curiosities where people try to scare you away to protect things. It is very toxic.” There are plenty of other mineral springs in the area that are safe to smell, swim in and explore.

More recently, scientists have entered the cave using protective measures, including a manhole blower to clear out the poisonous air. Since then, they’ve found new species and globally rare cave features formed by bacteria that consume hydrogen sulfide.

Hanging from the cave’s ceiling are snottites. While they look similar to stalactites, snottites are the consistency of mucus and formed by bacteria that metabolizes hydrogen sulfide. Snottites are found only in a handful of caves worldwide, with others located in Italy and Mexico.

Snottites and gypsum crystals hang from the ceiling of Sulphur Cave on Howelsen Hill.
Courtesy city of Steamboat Springs

“They’re similar to stalactites except that they are kind of slimy like snot, and they sway in the air and the wind,” said McKinley.

These bacteria excrete sulfuric acid, which can burn holes on skin and through clothing. As that sulfur dissolves other minerals in the cave, it forms crystals of gypsum.

Those sulfur-metabolizing bacteria also provide a food source for a globally unique worm, allowing them to survive in an environment without light and with little oxygen. These worms, Limnodrilus sulphurensis, are found only in Steamboat’s Sulphur Cave and are about an inch long — the width of a pencil lead — and appear blood red.

These worms, Limnodrilus sulphurensis, live only in caves in the Steamboat Springs area.
Courtesy city of Steamboat Springs

Their circulatory system allows the worms to absorb oxygen from the water in the low-oxygen environment. These factors have lead scientists to believe they could be useful in studying what can survive in low oxygen environments, such as the planet Mars, and to find medical benefits that could aid people with circulatory issues.

Over the years, multiple people have called for protection of the cave.

The National Natural Historic Landmark designation aims to encourage the preservation of sites that illustrate “the geological and ecological character” of the United States, enhance a location’s scientific and educational value by preserving it and to strengthen public appreciation of natural history, according to the National Park Service.

It would require the city, as the landowner of the cave, to “retain the integrity” of the cave as it stands now, according to the Park Service. Right now, the boundary of the landmark is proposed to enclose a few hundred square feet on Howelsen’s west side.

Under the designation, the city would retain ownership and management of the site, according to a presentation included in this week’s Steamboat Springs City Council agenda packet.

The presentation also includes possible management considerations that the city might take on. Snowmaking has occurred in the area for many years and is not believed to harm the cave. Excavation and grading within the boundaries of the landmark is the greatest threat to the cave, though sediment build-up from erosion in the water that flows into the cave could also harm the environment inside. 

Fourteen other sites are recognized with this designation in Colorado, including Hanging Lake near Glenwood Springs, Sand Creek at the Colorado Wyoming border south of Laramie, Wyoming, and Garden of the Gods in Colorado Springs.

“Not only is it worthwhile to give it the national recognition it deserves for being such a unique cave, but hopefully, it will bring awareness to other springs that we have in town that we don’t necessarily know that much about, that may have really unique qualities to them as well,” McKinley said. “We’ve already lost a couple of really important springs in Steamboat: the Soda Spring and the famous Steamboat Spring, which was once considered to be the only natural geyser in Colorado. All of the springs we have have their own value even if we don’t necessarily know what it is, so it’s really important to preserve the ones that we do have left.” 

To reach Eleanor Hasenbeck, call 970-871-4210, email or follow her on Twitter @elHasenbeck.

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