In 2014 a mysterious crater suddenly appeared on the Yamal peninsula in north-west Siberia. The debris surrounding this 50-metre-deep hole suggested it had been produced by an explosive process. Since then, scientists and local people have discovered several more craters on the Yamal and nearby Gydan peninsulas and a multitude of explanations have been put forward, ranging from meteor impacts to natural gas explosions. Now a new study has revealed the cause.

Drill down through the seasonally frozen soil in this region and you reach a thick clay permafrost layer. Sandwiched between the soil and permafrost lie unusual metre-thick ponds of very salty water known as cryopegs, which are underlain by crystalised methane-water solids, kept stable by the high pressure and low temperature.

Researchers found that warmer temperatures and longer summers in recent decades had resulted in the soil layer defrosting to greater and greater depths. Their results, published in Geophysical Research Letters, show that when the thaw reaches a cryopeg, the pressure from added meltwater forces cracks to open in the soil above. The new cracks create a sudden drop in pressure, which destabilises the methane hydrate and releases an explosive bubble of methane gas.

Although infrequent, the explosions release large amounts of methane and could have a significant warming impact.