It is known as “the Gateway to the Underworld” by local people who fear to go near the massive crater that suddenly appeared in the frozen heart of Siberia. And they are right to be afraid. For as the permafrost melts, the world’s biggest “megaslump” is expanding rapidly.

Already about a kilometre long and 90m deep, it is widening by up to 20m a year, making walking near its precipitous edges a dangerous pursuit. But Batagaika crater, which first appeared about 25 years ago, is also a sign of the rate at which the world is warming – smaller ones have been appearing increasingly across the northern hemisphere. FULL REPORT