At first, Andrew Christ was ecstatic. In soil taken from the bottom of the Greenland ice sheet, he’d discovered the remains of ancient plants. Only one other team of researchers had ever found greenery beneath the mile-high ice mass. But then Christ determined how long it had been since that soil had seen sunlight: Less than a million years. Just the blink of an eye in geologic terms.

And it dawned on him. If plants once grew at multiple spots on the surface of Greenland, that meant the ice that now covers the island had entirely melted. And if the whole Greenland ice sheet had melted once in the not-so-distant past, that meant it could go again.

“Oh my god,” he thought.

The findings, published Monday in the Proceedings of the National Academy of Sciences, indicate that the biggest reservoir of ice in the Northern Hemisphere can collapse due to relatively small increases in temperature over a long period of time. That makes it even more vulnerable to human-caused warming, which is causing the Earth to warm faster now than at any other period in its history.

Greenhouse gas emissions from human activities have already raised global average temperatures more than 1 degree Celsius (1.8 degrees Fahrenheit) since 1880. Greenland is losing ice at its fastest rate since humans invented agriculture, causing about 14 millimeters of sea level rise in the past half-century.

If the island’s entire ice sheet were to melt now, global sea levels would rise by more than 20 feet. “We don’t want to see what that looks like,” said Christ, a geologist at the University of Vermont. “It underscores the urgency of needing to change the way things are going right now.” The story of this soil sample is almost as dramatic as the data it contains. It comes from the bottom of an ice core taken during “Project Iceworm,” a failed Cold War effort to hide nuclear missiles beneath Greenland’s ice.

Camp Century, in the far northwest of Greenland, was to be a base for the U.S. military project. Housing, dining and medical facilities, all powered by a nuclear reactor, were dug into the ice.

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In the “battle between man and nature … man has brought his greatest scientific achievement — power from the atom — to the very top of the world,” broadcaster Walter Cronkite declared during a visit in 1960. “But can he live here? Can he stop the crushing force of the ageless ice?” To disguise the true purpose of the venture, the United States solicited scientists to conduct research at the site. Among the experiments was a first-of-its-kind project to obtain an ice core that spanned the entire depth of the ice sheet.

“That ice core revolutionized our understanding of Earth’s past climate,” Christ said. By measuring the types of oxygen contained within each layer of ice, researchers could get a rough estimate of how warm it was when the water froze. Analyses of ice cores from Greenland and elsewhere have allowed scientists to reconstruct a record of global temperatures going back tens of thousands of years. But the roughly 12 inches of soil from the bottom of the Camp Century ice core was never studied.

Meanwhile, the folly of Project Iceworm became apparent when the ice began to shift. Tunnels collapsed. Equipment got crushed. The nuclear reactor was swiftly dismantled, and the camp abandoned. Any scientific materials collected were sent off to laboratories and rarely thought of again.