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My Google Earth mud volcano map of Azerbaijan gives you and idea of just how ludicrously many mud volcanoes there are both onshore and offshore.

While it is unclear how a mud volcano can catch fire naturally, one theory is that it might happen when a mixture of flammable gasses is ignited by sparks from rocks colliding together.

"The mud volcanoes in Azerbaijan are some of the biggest and most violent in the world. There are, on average, several large mud volcano eruptions each year, and many of them can have big fires," Dr Mark Tingay, a geophysicist at the University of Adelaide, wrote on Twitter.

The blast, which erupted in an area full of oil and gas fields on Sunday, was caused by a mud volcano, the government says.

It said the fire had been caused by a mud volcano, which spews both mud and flammable gases.