If you have ever driven on I-70 near Denver, you have almost certainly passed the small and charming town of Idaho Springs and the eye-catching mountainside Argo Mine. While the building itself seems solid and in-tact, there is one part of it that few have seen, as it is decaying and becoming reclaimed by Mother Nature:

Located 4 1/2 miles from the Idaho Springs gold mill, the Argo Tunnel transported the mine's drainage before being condemned by the U.S. Environmental Protection Agency (EPA).
Wikimedia Commons/Plazak
Once a vital part of mining operations, the old Argo Tunnel delivered diverted water via its underground passageways and also provided another point of entry for the mines.
Wikimedia Commons/Unknown author
Unfortunately, the Argo Tunnel did more than it was supposed to, as its drainage was full of toxic metals and severely polluted nearby Clear Creek (pictured), turning the stream orange from the so-called acid water.
Wikimedia Commons/Plazak

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Though mining operations ceased for good during WWII, the Argo Tunnel still underwent renovations and upgrades to keep it from leaking toxins into nearby clean water, a la a nearby water treatment facility, and a high-pressure bulkhead installed near the entrance.
Flickr/Chris-Håvard Berge
Today, the Argo Tunnel looks eerie yet beautiful as nature has started to reclaim this once-prosperous area.
Flickr/Wally Gobetz
While the Argo Tunnel is closed, the Argo Mine is still open for tours, gold panning, and shopping.
Flickr/Wally Gobetz
To learn more about the Argo Mill - or to learn more about tours - please visit the Argo Mine's website.
Wikimedia Commons/Craig Talbert

Do you have a favorite attraction, event, or restaurant that you would like to see featured here on Only in Colorado? Nominate it here! For even more abandoned places around Colorado, check out These 16 Abandoned Places In Colorado Will Give You Goosebumps.

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Argo Tunnel Mine, Colorado 80452, USA