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When a Hole in Your Backyard Turns out to Be a 900-Foot Deep Mineshaft

(Photo: Tom Malloy)

Every spring in Butte, Montana, it's "shaft season." That's when old mineshafts make their appearance in the ground, often in surprising locations.

In the 19th and 20th Centuries, Butte was a major copper mining center. People dug hundreds of shafts deep in the the ground. When the copper veins empied out, the owners covered them up and went elsewhere.

So with the spring thaw, sinkholes form in the ground over these old mineshafts, some of which are several hundred feet deep. When they do, people call Tom Malloy, the county reclamation manager. Atlas Obscura talked to Malloy about how he deals with these mineshafts:

“The ones that turn up were the ones that were dug in the 1880s, 1870s, back before they were well regulated,” says Malloy. “They’re not marked on a map, which would be really nice. But they weren’t required to do that. Those end up being a surprise when they open up. You don’t know if they go down 10 feet and stop, or 1,000 feet down.”

Every spring, Malloy ends up investigating 50 to 100 reports of possible shafts, called in by city residents worried about a bit of sinking earth on their backyard or street. First, he and his colleagues will check old maps to see if there’s any indication that a shaft was once dug in that spot. They’ll check whether the alleged shaft is in the historic mining district, and if there have been any other mine shafts found in the vicinity. If they think it’s possible they might be looking at mine shaft, they’ll get the backhoe and do a little test digging to open it up and see what it is. Mine shafts aren’t the only things hiding in Butte’s ground: outhouses sometimes begin to sink into the earth when the ground thaws in the spring.

“There’s no way of knowing until you dig a hole,” says Malloy. “The most recent one, I would have bet $50 that it was an outhouse, but I was proved wrong. I don’t guess anymore, because I’ve been wrong so many times.”

If the backhoe starts bringing up residential knickknacks and materials, that’s an outhouse. If it starts hitting big, structural timbers, Malloy says, that’s a mine shaft.

-via David Plotz

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