Don’t Go Inside Iowa’s Longest Tunnel If You’re Claustrophobic

Written by Kyle Glatz
Updated: August 2, 2023
© Ahmed Zaggoudi/
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Key Points:

  • The longest tunnel in Iowa measures approximately 150 feet in length, 40 feet in width, and 15 feet in height.
  • It allows vehicles to pass in both directions underneath the ridge where the roadway is constructed.
  • The Tunnel enables smooth transportation while maximizing the usage of available space.

Tunnel systems are great for transporting people under wide rivers or through snowcapped mountains, like The Anton Anderson Memorial Tunnel in Alaska. Iowa does not have high mountains that must be blasted through to cut down on travel time and danger. The state of Iowa also lacks large rivers through which commercial traffic must flow, so bridges are a better option for them. So, where does Iowa’s longest tunnel run?

Take a look at where this tunnel is located, how it was built, and what other tunnels exist in the state. People that don’t like small spaces will definitely benefit from avoiding this tunnel!

What Is Iowa’s Longest Tunnel?

The Harmon Tunnel is the longest tunnel in Iowa, measuring about 150 feet long, 40 feet wide, and about 15 feet high. This tunnel can facilitate two-way traffic as it passes underneath the ridge through which the roadway was built.

The interior of the tunnel allows people to walk on either side. That way, people that come to visit the Pammel State Park can pass through the tunnel and continue hiking or biking.

So, Iowa’s longest tunnel is very small compared to some of the longest tunnels in the United States. While short, the tunnel still has a very interesting history. First, let us find out where to find this structure.

Where is Harmon Tunnel Located?

Aerial drone image of farmland landscape in Iowa USA
The longest tunnel in Iowa is found in Madison County, Iowa.

©Felix Mizioznikov/

Harmon Tunnel is located in Pammel State Park in Madison County. That puts it just about four miles southwest of Winterset, a city in the area. People come from all over the state to go camping, stay in yurt cabins, and hike throughout the region.

Harmon Tunnel is the only highway tunnel in the entire state. The tunnels road is called Pammel Park Road, and it runs to IA 322 if traveling west to east. Once the road passes through the tunnel, IA 322 runs north to IA 92.

The tunnel runs through a limestone ridge. The area above the tunnel isn’t perfectly settled, and a sign on the tunnel’s exterior warns of falling rocks. Also, the Middle River runs very close to the tunnel, so the tunnel can become impassible when the water level is too high. These hazards are a direct result of where and how the structure was built.

At first glance, one might believe that the tunnel was built in the middle of nowhere. They are correct. Then again, builders did not construct the tunnel to carry cars. When we consider the history of the tunnel, it becomes clear why it was constructed in this location.

When Was Iowa’s Longest Tunnel Built?

Middle River. Iowa. USA.
The Harmon Tunnel diverted some water from the Middle River for use in a sawmill.


Iowa’s longest tunnel was first built in 1858. Originally, a man named William Harmon along with his three sons built the tunnel. Their goal was to break through the soft black shale of the ridge to connect their sawmill with the Middle River. That way, they could use the water to power the mill. We do not know how long it took for the four men to dig the reported 150 feet through the ridge. Eventually, they completed the millrace. However, the millrace that they dug was not the same size as the tunnel today.

So, 1858 would not be the last time the tunnel people would transform the tunnel. The Harmon’s sawmill would eventually become a grist mill. Eventually, the idea of a mill was abandoned there altogether in 1904.

At some point in the early 1900s, some Iowa roadworks agencies saw the potential of the tunnel and started the process of widening it. By 1925, Harmon Tunnel became the first and only highway tunnel in the state. Just three years later, the Pammel State Park opened.

That was not the end of the tunnel’s construction, though. The tunnel also received some sort of expansion or fortification in 1982. Nowadays, the tunnel is wide enough to carry two lanes of traffic as well as a sidewalk on each side. However, the tunnel is only 15 feet high at its greatest measure. That means cars, vans, SUVs, and smaller passenger trucks will fit, but little else.

What Wildlife Lives Near the Harmon Tunnel?

Red-eared Slider
Red-eared sliders are common turtles throughout Iowa.

©effective stock photos/

The longest tunnel in Iowa is located near a river and sits amid a state park. Needless to say, many animals live in this area. Some of the wildlife that lives in this region includes:

This list should give you an idea of the wide assortment of animals that thrive in the area. A visitor may not spot all of them within the park, but they are common in this part of Iowa. People can find a lot more different animals living in the region as well. That’s especially true if a person is looking at the various fish that thrive in the Middle River.  

What Other Tunnels Exist in Iowa?

The surface of the stalactite in the limestone cave.
Iowa has several caves and a few man-made tunnels that were used for storage in the past.


Although Harmon Tunnel is the only highway tunnel in the state, it is not the only tunnel in the state. In fact, Iowa City, many tunnels and caves pass beneath the city. Some caves and tunnels were used to travel between buildings while others were small tunnels built to lead into and link together storage rooms.

Some rumors claim that the tunnels were once used as a part of the Underground Tunnel, moving people from place to place until they reached a safe haven. Yet, none of those tunnels were nearly as long as the Harmon Tunnel.

The longest tunnel in Iowa was created to help divert water from the Middle River to power a mill. Now, it is a roughly a 150-foot-long tunnel that was designed to help people travel through Pammel State Park and reach a highway to the north. This tunnel may not be as large as those found in other states, but at least it has an interesting story about its creation.

Where is The Harmon Tunnel Located on a Map?

The Harmon Tunnel holds the distinction of being the longest tunnel in Iowa, with dimensions of approximately 150 feet in length, 40 feet in width, and standing about 15 feet high. It serves as a crucial pathway for two-way traffic, allowing vehicles to smoothly navigate beneath the ridge.

Here is the Harmon Tunnel on a map:

The Featured Image

flag of Iowa
Iowa Waving Flag in 3D
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About the Author

I've been a freelance writer since 2013, and I've written in a variety of niches such as managed service providers, animals, and retail distribution. I graduated from Rowan University in 2014. When I'm not working, I enjoy playing video games, reading, and writing for fun.

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