Mud volcanoes: What are they and how do they catch fire?

An enormous fireball lit up the night sky above the Caspian Sea, just south of Azerbaijan, caused by a mud volcano on Sunday night.

There have been no injuries reported from the blast and the Azeri state oil company, SOCAR, has said there has been no damage to their oil farms.

The explosion is thought to be natural, as the region is home to lots of mud volcanos. Although they do not have lava like a regular volcano, they can still catch fire once they erupt.

Mark Tingay, a geophysicist from the University of Adelaide, said: "Azerbaijan has basically the perfect geological conditions for mud volcanoes".

A mud volcano is similar to a normal volcano, but they don't have any lava flowing through them.

They come in different sizes, some as small as just a few centimetres wide and others as big as hundreds of metres high, and they can form underwater in the sea as well as on land.

They are caused by water being heated very deep within the Earth and that mixes with rocks and minerals - when they erupt, this mixture is forced to the surface and can catch fire.

It's currently unknown how a mud volcano can catch fire naturally but one theory is that it might happen when a mixture of flammable gasses is ignited by sparks from rocks colliding together.

The suspected mud volcano in question is thought to be the Makarov Bank.

The mud volcano previously erupted and released flames in 1958, where the fire reached heights of 500-600 metres high and was 150 metres wide.

It's located in the Caspian Sea, which is just south of Baku - the capitol of Azerbaijan.

Azerbaijan has been known as the "land of fire" because of the oil that sits below the surface in certain areas as well as natural gas reserves.