NOVA 49x5 - Arctic Sinkholes

Colossal explosions shake a remote corner of the Siberiantundra, leaving behind massive craters. In Alaska, a hugelake erupts with bubbles of inflammable gas. Scientists arediscovering that these mystifying phenomena add up to a tickingtime bomb, as long-frozen permafrost melts and releases vastamounts of methane, a potent greenhouse gas. What are theimplications of these dramatic developments in the Arctic?Scientists and local communities alike are struggling to graspthe scale of the methane threat and what it means for ourclimate future.