Entry No. 39 — the Des Moines file — filed under: only-highway-tunnel
Harmon Tunnel at Pammel State Park
The Harmon Tunnel is Iowa's only auto tunnel — a 150-foot passage hand-dug through a solid limestone ridge in 1858 as a mill-water chute, later widened for cars, where you actually drive your vehicle through the dark heart of the Devil's Backbone. The tunnel is still the primary road into the rest of Pammel State Park, so every visitor passes through it.
The story
In 1858, long before anyone thought about paved state park roads, workers pitched hand tools against solid limestone to hack a 150-foot mill-water chute straight through a narrow ridge known as the Devil's Backbone. Decades after the mill fell quiet and cars replaced horses, local road builders realized they didn't need to bypass the ridge—they just needed to widen the hole. They blasted out enough rock to clear a chassis, turning a pioneer water race into Iowa's first—and only—highway tunnel.
When Pammel State Park was dedicated in 1928 as one of Iowa's earliest state parks, Harmon Tunnel was preserved as its primary entrance. You don't take a scenic bypass to see it; you are forced to drive straight through the limestone belly of the ridge just to get to the campgrounds and riverbanks on the other side. It remains a raw 150-foot tube of hand-worked rock, proving that sometimes the shortest path between two points in Iowa is right through a cliff.
What it's like
Approaching the entrance feels less like entering a park and more like driving your sedan into a cave mouth. The passage is narrow and technically handles two-way traffic, so creep along at low speed and keep your eyes open for oncoming cars or hikers using the claustrophobic pedestrian shoulders. Kill your brights, roll your windows down, and drop your speed: the temperature inside drops immediately, smelling of wet rock and river mud, while your engine note bounces off the rough walls in a loud echo. Once you pop out the other side, turn right into the Middle River Ford parking lot to scramble up the Devil's Backbone trail and look down at the ridge you just drove through.
The move: Drive slowly through the tunnel together (roll the windows down to feel the cool limestone air and listen to the echo), then hike the Devil's Backbone trail above to look down at the ridge you just drove through — finish with a picnic on the Middle River bluffs at sunset.
📍 Before you go Pammel sits about four miles southwest of Winterset — head west on Highway 92, then south on County Road P68. You don't have to seek the tunnel out; it's the only road into the park, so you'll roll through it on the way in. The passage is two-way and narrow with pedestrian walkways, so creep along and watch for hikers. Spring through fall is best for the trails, the limestone bluffs, and the Middle River ford crossing (impassable in high water and closed in winter). Pair the trip with Winterset's Madison County covered bridges, the courthouse town square, and the John Wayne Birthplace Museum.
- 📍 Winterset
- 💸 Free
- ⚡ Up for anything
- 🌗 Outdoors
Where: 1900 Pammel Park Rd, Winterset, IA 50273
Hours: Mined 2026-06-07 — confirm current hours before you go.
Pair it with
- Clark Tower — Double down on Madison County's strange stone structures by driving four miles northeast to climb a 25-foot limestone castle hidden in the woods.
- PepperHarrow Farm — Balance the subterranean chill of the tunnel with a three-mile drive to wander open fields at a family-run lavender and flower farm.
- Briar Patch Amphitheater — Head 15 miles east to trade a tunnel carved for a 19th-century mill for a natural bowl amphitheater carved into an oak hillside by a Grateful Dead fan.
Good to know
Is Harmon Tunnel really the only auto tunnel in Iowa?
Yes. While Iowa has railroad tunnels and pedestrian underpasses, Harmon Tunnel is the state's sole highway tunnel carved for motor vehicles.
Is the tunnel wide enough for two large vehicles to pass inside?
It is extremely narrow. While it serves two-way park traffic, drivers usually yield or take turns creeping through, especially if pedestrians are using the walkways.
Can you cross the Middle River right after emerging from the tunnel?
Only when water levels allow. The famous concrete ford just past the tunnel is impassable during high water and closed entirely during the winter.
Plan a visit & invite your people →
Further reading: khak.com · madisoncountyparks.org · notesoniowa.com · fieldtripiowa.com
last checked: 2026-06-07