The Gobustan Mud Volcanoes are twenty-odd cones of cold grey mud on a cracked plain by the Caspian Sea in the Qaradağ district south of Baku, in Azerbaijan, driven not by heat but by gas pushing clay and brine up from below, and they are walked among on the plain, watched one at a time from a few metres back as they gurgle and spit, and climbed for the sea on the horizon. Azerbaijan holds nearly half the mud volcanoes on Earth, some four hundred, because the country sits on the oil and gas that push them up, and where a mud volcano stands, oil usually lies far beneath it, which is how the industry followed the mud; the cones at Dashgil are a metre or a few metres high, each with a crater of grey slurry that bulges, breaks and slumps as a bubble of gas comes through, cold to the touch and slow enough that a single crater is timed from five metres back, and where the mud spilled out and dried the surface has shrunk into a tight mosaic of polygonal plates that crunch and tilt underfoot. The cones only gurgle, but the big ones of the region catch fire, when the gas ignites and throws flames hundreds of metres up, as one did in the Caspian in 2021.
Planetary scientists come to these flows to understand Mars, where landforms long read as lava are now thought to be mud that erupted much the same way, and the field is reached by a rough track from the main road. A marked track runs out across the plain for quad bikes between clusters of cones the footpaths never reach, and the taller cones at the far edge are enough for the Caspian on the horizon. For a few weeks in spring the bare semi-desert greens and flowers with steppe irises, and the rock engravings of Gobustan are on the same road. The Gobustan volcanoes are the earth belching cold mud, and the oil underneath is the reason.