Long-lost Core Drilled to Prepare Ice Sheet to Hide Nuclear Missiles Holds Clues About a Different Threat Plant remnants frozen beneath 4,000 feet of ice show that most of Greenland’s ice sheet melted about 1 million years ago, in a climate like today’s. March 15, 2021 The Greenland Ice Sheet, which has enough frozen water to raise sea levels by 20 feet, melted away completely at least once about 1 million years ago, new research shows. Credit: Joshua Brown Related Share this article Before the U.S. military tried to hide nuclear missiles under the Greenland Ice Sheet during the Cold War, they asked scientists to determine the stability of the ice that would shelter the warheads. From the secret nuclear-powered Project Iceworm base, a maze of ice tunnels and caves spanning nearly two miles, they drilled down more than 4,000 feet to the ground, retrieving the world’s first deep ice core sample from a polar ice sheet in 1966. Today, those once-forgotten ice cores are advancing the understanding of a different, but no less global threat than nuclear war—climate change.