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 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.
Plan a visit & invite your people →
Proof: source 1 · source 2 · source 3 · source 4
last checked: 2026-06-11