Human activity contaminated this glacier before anyone stepped foot on it

Researchers studied an ice sample that was extracted from a glacier on the highest peak ever drilled. The site on Mount Shishapangma sits at 23,600 feet above sea level.

When researchers analyzed and dated the ice, they found unnaturally high levels of toxic metals that are associated with coal combustion. They discovered that this contamination started in the 1780s during the Industrial Revolution.

The contaminants are likely ash from London factories, or residue from forest fires set by farmers in order to clear land for a growing population.

The obvious question is how the ash found its way from its place of origin into this glacier.

Paolo Gabrielli, the study's lead author and a researcher at The Ohio State University, thinks that the ash entered the upper atmosphere. From there, winds that travel around the globe from West to East blew it all the way to the Himalayas. Trace metals from the ash contaminated snowflakes, which then fell onto the mountain.

No human stepped foot on the mountains surrounding this one until 1953. Due to the glacier's height, the contaminants are more likely to be from higher up in the atmosphere than from the nearby ground.

"Ice samples from polar regions tell stories that have a global significance, because polar regions are so far from local emissions like villages, industries and forests," Gabrielli explained.

The ice core analyzed in this study was collected in 1997 by an international team of researchers, and preserved in a freezer at The Ohio State University's Byrd Polar Research Center. It has been the subject of many experiments since then, as growing technology has allowed for increasingly complex analyses.

Ice cores are cylinders of ice that are drilled out of a glacier, formed by snow buildup over thousands of years.

Lonnie Thompson, one of the researchers who helped drill this core, says ice cores like these are "frozen archives of the past."

"They contain a history of the temperature of the planet," he said. "All the greenhouse gasses are trapped in the little bubbles, so we can look at what the natural variability was, long before humans ever became a factor."

He says preserving ice is especially important today, because in a few years, these archives could all be gone.

"A lot of the glaciers that I drilled early in my career are no longer out there," Thompson said. "In a way, our program has become more of a salvage mission to capture these records before they are compromised by melting."