You’re driving through northwest Iowa. It’s flat. Like, really flat. Cornfields stretch out in every direction until the sky swallows them. You pass a sign for a town called Manson. It looks like every other quiet community in the Midwest, but right under your tires, there’s a scar in the earth so massive it’s hard to wrap your head around.
It’s the Manson Crater, or as geologists call it, the Manson Impact Structure.
Most people have no clue it’s there. Why? Because you can’t see it. There’s no gaping canyon or jagged rim. Instead, there’s about 100 feet of glacial till—basically a thick blanket of dirt and rocks left by the last Ice Age—hiding a 24-mile wide disaster zone.
The Day the Sky Fell on Iowa
Imagine Iowa 74 million years ago. It didn't look like this. It was the shore of a shallow sea called the Western Interior Seaway. No corn. No tractors. Just dinosaurs and giant marine reptiles.
Then, a rock the size of a mountain—roughly 1.2 to 2 miles wide—screamed into the atmosphere. It was moving at maybe 45,000 miles per hour. When it hit, it didn't just make a hole. It vaporized the ocean and the ground instantly.
Geologist Raymond R. Anderson, who literally wrote the book on this thing (The Manson Impact Structure, Iowa: Anatomy of an Impact Crater), says the blast was like 10 trillion tons of TNT.
Think about that.
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An electromagnetic pulse probably fried everything for 130 miles. If you were standing where Des Moines is now, you’d have been incinerated before you even heard the sound. The ground liquified. It turned into a "cauldron of molten rock" wider than an entire Iowa county.
The Biggest Case of Mistaken Identity in Geology
For a long time, the Manson Crater was the "prime suspect" in the greatest whodunnit in history: the extinction of the dinosaurs.
Back in the 1980s and early 90s, scientists were desperately looking for the crater that killed off T-Rex. They knew a massive impact happened right at the end of the Cretaceous period (the K-Pg boundary). Manson was the right size. It was in the right place. For a hot minute, Iowa was the center of the scientific world.
In 1991 and 1992, the U.S. Geological Survey and the Iowa Geological Survey started drilling. They pulled up cores of "shocked quartz"—sand grains that have been physically deformed by the pressure of a massive explosion.
But then the lab results came back.
Using argon-isotope dating, they found out the Manson impact happened about 74.1 million years ago.
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The dinosaurs died 66 million years ago.
Manson was 8 million years too early. It was a massive, world-altering event, but it wasn't the event. The real killer was the Chicxulub crater in Mexico. Manson was just a very violent prequel.
Why the Water in Manson is Weird
Even if you can’t see the crater, you can taste it. Sorta.
Manson is famous for its "soft water." In most of Iowa, the water is incredibly hard because it’s full of calcium and magnesium from limestone. But inside the Manson Crater, the impact was so violent it shattered the limestone and pushed up Precambrian granite from three miles deep.
When the town drills wells, they aren't hitting the usual Iowa layers. They’re tapping into a unique aquifer created by the impact rubble.
In 1905, people in Manson realized their water was different. It didn't leave scale in the boilers. It felt different on the skin. For years, Manson called itself the "soft water capital of the world."
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They didn't know they were drinking "meteorite water" until geologists connected the dots decades later.
Can You Actually Visit the Manson Crater?
If you go to Manson today expecting a scenic overlook, you’re going to be disappointed.
You’ll see a flat horizon. But the town has embraced its "impact" identity. There’s a big sign as you enter that says "Manson: Making an Impact."
The local library has an exhibit with some of the drill cores and rocks that were pulled from the depths. It’s one of the few places on Earth where you can touch stones that were literally melted by an asteroid.
Why It Still Matters
Manson is one of the best-preserved complex craters in the world. Because it was buried by glaciers shortly after it formed (geologically speaking), the structure hasn't been eroded away like many others. It’s a "window" into how these massive impacts work.
Scientists like Christian Koeberl have used the data from Manson to understand craters on Mars and Venus. It's a laboratory hidden under a cornfield.
What Most People Get Wrong
- Myth: You can see the rim from a plane. Reality: You can't. The glaciers leveled everything.
- Myth: It killed the dinosaurs. Reality: It happened millions of years before the big extinction.
- Myth: It’s just a small hole. Reality: It’s 24 miles wide. If you put the center in downtown Manson, the edges would reach nearly to Fort Dodge.
Actionable Insights for Your Visit
If you’re a geology nerd or just like weird history, here is how to actually "see" the crater:
- Check out the Manson Public Library: They have the most accessible information and some physical samples of the impact breccia.
- Look for the "Making an Impact" sign: It’s the classic photo op on the edge of town.
- Visit the Lizard Creek area: There used to be an outcrop of "shocked" shale near the bridge, though much of it has been covered or destroyed by construction over the years.
- Drive the diameter: Start in Manson and drive 12 miles in any direction. Realize that everything you just passed was once a literal hole in the earth three miles deep.
The Manson Crater is a reminder that the ground beneath us isn't always as solid—or as boring—as it looks. Sometimes, the biggest stories are the ones you have to dig for. Literally.