'Zombie fires... were once a rare occurrence in the boreal regions that stretch across the far north through Siberia, Canada and Alaska. But in a rapidly heating world, they are becoming increasingly common. The overwintering burns are small... but they are transforming fires into multi-year events'

Los Angeles officials theorise that a small fire burned its way down tree roots and remained there smouldering even when firefighters on the surface believed that it had been extinguished, only to later break out and grow into an inferno