Photograph by Flickr user Proper Pictures.

Living near an active volcano can be a little anxiety-producing, but it's not the only fire raging below the Earth's surface. Coal is a long-burning mineral hidden underground that can ignite when the circumstances are right. Those circumstances include contact with air, which happens when we try to take the coal out of the ground for our own purposes. Here are five coal seams that have been burning for a long time.

1. Brennender Berg, Germany

Photograph by Kh80.

Brennender Berg in English is "Burning Mountain," although it is more of a gorge than a mountain. It refers to an area of Germany between the towns of Dudweiler and Sulzbach-Neuweiler, under which a seam of coal has been burning for more than 300 years. In 1688, the tale goes, a shepherd started a wood fire on a tree stump, which burned to the roots and ignited the coal seam. More likely, this was a case of spontaneous combustion. The burning coal seam is now evidenced by glowing embers and sulfurous smoke, rather than flames as in the past. The area is not as dangerous as it was in previous centuries, and is open for visitors. A hike is recommended for days after a rain.

2. Smoking Hills, Canada

Photograph by Ansgar Walk.

The Smoking Hills of Canada are located near Franklin Bay on the Arctic Ocean in the Northwest Territories of Canada. The uninhabited hills (the nearest community is about 65 miles away) are located above the Arctic Circle. Discovered in 1826 by explorer John Franklin, the hills are notable for exuding constant smoke from burning coal underground. The mountains contain large deposits of lignite (brown coal), sulphur, and oil shales. A reaction between sulphur and lignite leads to spontaneous combustion, and it is believed that the lignite has been burning for hundreds of years. The smoke coming from the coal seams is highly acidic, and the lakes surrounding the Smoking Hills are polluted with sulphur dioxide, meaning they're habitable only by species that thrive in acid environments. In 1850, Robert McClure's expedition into the Northwest territory took them to the Smoking Hills. They collected a specimen of the burning minerals, which burned a hole through the captain's mahogany desk. The various chemicals in the burning rock and smoke have left bright stripes of color along the hills.

3. Burning Mountain, Australia

Photograph from Upper Hunter Tourism.

Officially named Mount Wingen, Burning Mountain is located 139 miles north of Sydney, Australia, and is part of the Burning Mountain Nature Reserve. The mountain was first thought to be a volcano because of the vents spewing sulfurous smoke, but later investigation proved the mountain actually has a burning coal seam running underneath. The traditional story told by the people indigenous to the area centers on a woman whose husband did not come home from a battle. Wanting to die herself, she climbed the mountain, but was instead turned into a stone that cried tears of fire. These burning tears rolled down the mountain and set it aflame. Today, scientists thinks the fire started either after a lightning strike, or by spontaneous combustion.

The mountain's coal seam is two meters thick and 20-30 meters below the surface. Estimates of how long it has been burning range from 5,000 to 15,000 years.

4. Centralia, Pennsylvania, USA

Photograph by Flickr user Scott Drzyzga.

Coal-rich Pennsylvania has endured plenty of coal seam fires, but most eventually are extinguished or burn themselves out. Not so in the town of Centralia. Deep mining left passageways underground as strip mining tore up the surface above. As the coal ran out, the town was left to clean up the pits left behind. In 1962, volunteer firefighters ignited a fire to clean out one pit-turned-landfill in order to make room for more garbage. The fire was doused with water that night, but was found to be burning the next day. More attempts to put the fire out were unsuccessful, and it was discovered that a hole in the pit led to a mine passage. That started the fire down a coal seam, which stymied all attempts to extinguish it. However, others theorize that the coal seam fire already existed before the landfill burning. At any rate, the fire continued for years, long after the town ran out of money to fight it. The underground fire reached downtown by 1979, when a gas station owner (who was also the mayor) measured his gasoline storage tank at 172 degrees! In 1981, a 12-year-old boy fell into a sinkhole, releasing steam and carbon dioxide (he was pulled out by his cousin and survived). The citizens of Centralia abandoned the the unsafe town, accepting government buyouts on their property offered in 1984. The nearby town of Byrnesville was also abandoned because of the underground fire. Centralia was officially condemned in 1992, but a very few residents refused to leave. The 2010 census listed the population of Centralia at ten people. The fire has now been burning for over 50 years.

5. eMalahleni, South Africa

Photograph from eo miners.

Coal mining in the Mpumalanga province (formerly East Transvaal) of South Africa centers around the city of eMalahleni, which means "place of coal." It was formerly known under its Afrikaans name, Witbank. The extensive Transvaal and Delagoa Bay Collieries were abandoned in 1953 without reclamation, and parts of the mines have been on fire ever since. Witbank coalfield mines typically had a low coal recovery ratio, and the pillars of coal left behind spontaneously combusted in contact with oxygen. The abandoned underground mine passages collapse unexpectedly, and have injured and killed people occasionally over the years, either from the collapse or the burning coal underneath.

These are not the only long-burning coal seam fires. They've happened throughout history, and sometimes burn for centuries. There are other underground coal fires burning today as well, all over the world.