An enormous explosion that was caught on camera in the Caspian Sea on Sunday has been blamed on a phenomenon known as a mud volcano. The massive blast sent a column of fire into the sky in an area of the sea where Azerbaijan runs a lot of offshore oil and gas fields. But the state oil company said it wasn’t to blame for the explosion—and that it was caused by a mud volcano spewing flammable gas into the sky. According to The Guardian, the volcanic activity can cause rocks to hammer into each other and create sparks that can ignite the gasses. Mark Tingay, a mud volcano expert at Australia’s University of Adelaide, said Azerbaijan’s explanation is plausible, saying the explosion “certainly could be a mud volcano” and that the location “fits roughly” with a known volcano complex.