What happens when the Arctic’s ancient freezer starts to fail? Scientists have uncovered the truth behind giant sinkholes exploding from the frozen ground in Siberia, and a vast lake bubbling with methane in Alaska .They are pointing to a dramatic shift beneath our feet – one driven by thawing Arctic permafrost, the vast frozen layer that stores nearly twice as much carbon as the entire atmosphere. Learn why these geological curiosities reveal a dangerous climate wildcard – one powerful enough to accelerate global warming in ways current models don’t yet predict.To watch the full film, visit https://www.pbs.org/wgbh/nova/video/arctic-sinkholes/Learn more about NOVA and subscribe to our YouTube channel.