Tucked in a perfect little grotto behind a waterfall in New York's Chestnut Ridge County Park sits an extraordinary phenomenon - an eternal flame. Fuelled by the gas that leaks out from cracks in the Earth’s surface, this 20-centimetre-high flame is said to have been lit by the Native American residents of the area many hundreds, or possibly thousands, of years ago.
There are hundreds of 'natural' eternal flames burning around the world, but the Chestnut Ridge flame is particularly special, according to Douglas Main at LiveScience. Usually, an eternal flame will feed on the natural gas that forms deep underground, emerging around ancient and super-hot deposits of shale - a common type of sedimentary rock made of hardened mud and clay. Ordinarily, these deposits need to be submerged in at least near-boiling water to produce the natural gas, but in the case of the Chestnut Ridge flame, the water surrounding these rocks is only lukewarm, making it the most unusual eternal flame ever found.
In 2013, geologist Arndt Schimmelmann from Indiana University in the US decided to investigate this strange flame. He found that not only was the underground water beneath it barely warm enough to rival a cup of tea - and yet still able to produce enough natural gas to keep the flame alight - the shale rocks around which the gas is produced were found to be significantly younger than expected, and shallower.
Calling the Chestnut Ridge eternal flame "the most beautiful in the world”, Schimmelmann's analysis found that the gas that feeds it comes from a special type of shale called Rhinestreet Shale, which sits around 400 metres below the surface of the Earth. "Not only that, but it may feature the highest concentrations of ethane and propane of any known natural gas seep,” he said in a press release, adding that this unique flame happens to be fuelled by a previously unknown source of gas. "Approximately 35 percent of the gas is ethane and propane, as opposed to methane - the dominant constituent in natural gas. Ethane and propane can be valuable byproducts in the processing of natural gas," he said.
Schimmelmann published his observations in the journal Marine and Petroleum Geology.
And now to spoil some illusions: ‘eternal flames’ are unfortunately not as eternal as one might expect. They do go out occasionally, due to an ill-timed spluttering of the gas supply, or in the Chestnut Ridge’s case, too much water seeping in from the waterfall, but they only stay extinguished for as long as it takes for the next hiker to come by and relight it.