Underneath Azerbaijan is a lot of natural gas - the same kind of gas many kitchens use for cooking. In some places, the gas seeps up through cracks in the ground. If it meets a spark, it can light. Once lit, it just keeps on burning, because new gas keeps coming up from below.
Travellers and traders walking along the old Silk Road thousands of years ago saw these flames and were amazed. They told stories about a country where the ground itself was on fire. Today you can still walk right up to Yanar Dag and warm your hands at the flames, which are especially beautiful in winter when snow falls around them but never lands on the fire.
Azerbaijan's flag has a flame in its design, and the capital city of Baku has three modern skyscrapers shaped like giant flames, called the Flame Towers. At night the towers light up, copying the colour of fire. From across the bay, the whole skyline looks like it is dancing.
Scientists today understand exactly why the flames burn - it is the gas - but in the past people thought the fires were sacred. Old temples were built around some of them, including one called Ateshgah, where travellers used to stop and rest. The flames there are now kept alight from a pipe so the tradition continues even though the natural gas at that spot has run out.