In the Arctic, enormous releases of methane, a potent greenhouse gas, threaten the climate.

In the Arctic, enormous releases of methane, a potent greenhouse gas, threaten the climate. Colossal explosions shake a remote corner of the Siberian tundra, leaving behind massive craters. In Alaska, a huge lake erupts with bubbles of inflammable gas. Scientists are discovering that these mystifying phenomena add up to a ticking time bomb, as long-frozen permafrost melts and releases vast amounts of methane, a potent greenhouse gas.