Entry No. 11 — the Fort Collins file — filed under: toxic-cave
Sulphur Cave at Howelsen Hill
“The town ski hill has a cave that can kill you in two breaths.”
The town ski hill has a cave that can kill you in two breaths. Sulphur Cave drops about fifteen feet into travertine just west of the lower Mile Run, exhaling hydrogen sulfide so thick researchers enter only in supplied-air gear. Inside: dripping snottites of sulfur-eating slime and wriggling blobs of blood-red worms, Limnodrilus sulphurensis, that live nowhere else on Earth. The feds made it a National Natural Landmark in 2021. You stand at the fence, catch the rotten-egg breath, and peer in.
The story
Steamboat Springs built its reputation on winter sports and powder, but the base of the town ski hill hides something far less hospitable. Just west of the lower Mile Run trail at Howelsen Hill, a sinkhole drops fifteen feet into travertine rock. Inside, the geology vents a heavy mix of hydrogen sulfide and carbon dioxide gas so concentrated that stepping into the opening without supplied-air breathing gear can kill you in two breaths.
In 2021, the federal government named Sulphur Cave a National Natural Landmark, not for easy recreation, but for the rare, hostile ecosystem thriving in the dark. Below the surface, sulfur-eating bacteria form gooey, dripping stalactites known as snottites. In the toxic sludge beneath them, researchers found dense, wriggling clumps of *Limnodrilus sulphurensis*—a species of blood-red extremophile worm that lives nowhere else on Earth.
What it's like
Park near the rodeo grounds off Howelsen Parkway or take the footbridge across the Yampa River from downtown. You will catch the sulfur stench before you spot the fence surrounding the sinkhole. Stand at the barrier to look down into the travertine upper room, but take the warning signs seriously: do not cross the fence. The pool of toxic gas inside is invisible, trapped, and lethal.
The move: Walk over from downtown, follow the rotten-egg smell to the fenced sinkhole at the base of Howelsen, peer into the upper room together, then soak it off at Old Town Hot Springs back across the Yampa.
📍 Before you go The cave sits on city parkland at the base of Howelsen Hill, a short walk uphill from the rodeo grounds parking off Howelsen Parkway; look for the fenced sinkhole just west of the lower Mile Run ski trail. Do not cross the fence: the hydrogen sulfide and carbon dioxide inside are lethal within a couple of breaths, and entry is prohibited year-round except for permitted researchers with breathing gear. You will smell the sulfur spring before you see the opening. Downtown and Old Town Hot Springs are a flat walk back across the Yampa River footbridge.
- 📍 Steamboat Springs
- 💸 Free
- ⚡ Up for anything
- 🌗 Outdoors
Where: Howelsen Hill Ski Area, 845 Howelsen Pkwy, Steamboat Springs, CO 80487
Hours: Added 2026-06-11 — confirm current hours before you go.
Pair it with
- Howelsen Hill Ski Area — The cave sits right at the base of this ski area, so you can inspect lethal geology before riding North America's oldest continuously operating ski hill.
- Steamboat Spring & the Mineral Springs Walking Tour — Follow the geothermal system across the Yampa River to see how the valley's other mineral springs behave above ground.
- Sunpie's Bistro — Walk back into downtown Steamboat across the footbridge to wash the rotten-egg smell out of your nostrils with a cold drink by the river.
Good to know
Can you go inside Sulphur Cave?
No. Public entry inside the cave is strictly prohibited year-round. Lethal levels of hydrogen sulfide and carbon dioxide make entry dangerous for anyone without specialized, supplied-air breathing gear.
Where is the cave located?
It sits on city parkland at the base of Howelsen Hill Ski Area (845 Howelsen Pkwy), a short walk uphill from the rodeo grounds parking lot and just west of the lower Mile Run ski trail.
Why is the cave a National Natural Landmark?
Designated in 2021, the cave shelters a unique extremophile ecosystem, including sulfur-eating bacterial formations called snottites and *Limnodrilus sulphurensis*, a worm species found exclusively inside this single cavern.
Plan a visit & invite your people →
Further reading: steamboatpilot.com · smithsonianmag.com · cpr.org · Atlas Obscura
last checked: 2026-06-11