Credits: Courtesy of the researchers *Terms of Use: Images for download on the MIT News office website are made available to non-commercial entities, press and the general public under a Creative Commons Attribution Non-Commercial No Derivatives license. You may not alter the images provided, other than to crop them to size. A credit line must be used when reproducing images; if one is not provided below, credit the images to "MIT." Caption: Earth’s permafrost shifted to a more stable state in the last 400,000 years and has been less susceptible to thawing since then, according to a new study by MIT researchers and their colleagues, who are pictured here on a research expedition.