Scientists have discovered thousands of methane gas bubbles trapped underground in the Arctic, causing huge craters in the soil as the gas escapes from thawing permafrost. Many of the craters have been spotted by satellites, while scientists have launched expeditions to examine others. Experts are trying to map the locations of the methane bubbles to limit their impact on energy supplies and transport routes. Half the melting Arctic sea ice may be due to natural weather cycles rather than global warming, according to some scientists.