On the morning of August 15, 2015, excavator crews under contract with the Environmental Protection Agency ascended a rugged mountainside six miles north of Silverton, where they planned to carefully dig out the entrance to a century-old, heavily polluted gold mine and install a pump to draw down the contaminated water that had accumulated inside it.

What happened next would be described in minute detail in a 132-page technical evaluation released by federal officials two months later.

“As the excavator continued to dig … the operator reported hitting a ‘spring,’” the report read. “Within moments, the ‘spring’ began spurting upward 1.5 to 2 feet into the air.”

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“The EPA (on-site coordinator) reported that the water was initially clear and then turned red; in about 3 minutes it turned orange and the flow increased rapidly,” it continued.

Deep inside the Gold King Mine — as in thousands of other long-abandoned mines scattered across Colorado and other Western states — water had pooled in miles of subterranean tunnels and shafts, where it reacted with mining waste and naturally occurring minerals to become highly acidic. As it gushed out of the mine, the water swept away and dissolved toxic heavy metals like arsenic, lead and cadmium that had been the byproduct of decades of hardrock mining.

Long before its 2015 blowout, the Gold King Mine had been identified as one of Colorado’s most problematic sources of this so-called acid mine drainage. The state’s Division of Reclamation, Mining and Safety had worked with EPA officials for two decades to manage it, and in the mid-2000s, increased discharge levels, along with declining trout populations in the Animas River watershed, led to more active efforts at remediation. The EPA had successfully completed a “pump-down” operation at a nearby mine in 2011, and hoped to do the same at the Gold King.

The mine's problems persisted for years after the spill, prompting Colorado senators to file relief bills as recently as 2024 to address ongoing environmental and economic damage.

Instead, the botched excavation — conducted, the technical report found, without first drilling a borehole to determine the water level within the mine — caused one of the most notorious environmental disasters in Colorado history. The discharge from the blowout at the mine entrance didn’t subside for roughly an hour, resulting in the release of an estimated three million gallons of heavily contaminated water into Cement Creek, a tributary of the Animas River.

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A sickly yellow-orange plume drifted downstream for days following the incident, and impacts from the spill reached as far as New Mexico, Utah and the Navajo Nation, forcing many municipalities to temporarily find other sources of drinking water. Then-Colorado Governor John Hickenlooper and his counterparts in other states declared the affected watersheds a disaster area.

Earlier in Colorado’s history, gold and silver mining had been a major part of its economy — and long after that, the mining heritage and ghost towns of regions like the San Juan Mountains had been a cherished part of the state’s historical character. But the EPA’s blunder in 2015 brought forth a ghastly, mustard-colored reminder of that era’s toxic environmental legacy.

Production at the Gold King Mine had begun in 1886 and ended in 1923. It was a period during which environmental regulations on hardrock mining were nonexistent, though the harmful impacts of the industry’s waste were readily apparent.

“The city council met in special session tonight to appoint a committee to confer with the mill owners of Silverton regarding the polluting of the waters of the Animas River at that place with the tailings from their plants,” reported the Rocky Mountain News in May 1902, “and their rendering the same unfit for use as a water supply for Durango.”

Upstream mining interests fought back: “The alleged wanton pollution of the waters of the Animas River by the mill owners of San Juan County and the people of Silverton,” retorted the Silverton Standard, “is only in the jaundiced vision of the editor of the Durango Democrat.”

Lawsuits were threatened, but no agreement was reached. Durango abandoned its efforts to clean up the Animas watershed — which, San Juan mine owners were quick to point out, had been afflicted by a certain amount of naturally occurring contamination before their mills were in operation — and sought an alternative water supply instead.

Researchers would later estimate that more than four million tons of mine tailings were discharged into the Animas River watershed around the turn of the 20th century, greatly exacerbating the area’s naturally occurring acidic drainage problem.

A barrage of lawsuits against the EPA were settled in the years following the spill. Its long-term environmental effects are still being studied, and the chronic problem the agency was hoping to address in 2015 — the more gradual seepages of contaminated mine water flowing into the area’s watersheds — hasn’t abated.