Located in the desert of Al-Mahra province, the giant hole is 30 meters (100 feet) wide and thought to be anywhere between 100 and 200 meters (328 feet and 820 feet) deep. Local folklore says it was created as a prison for the demons, a reputation bolstered by the foul and toxic odors rising from its depths. Some have called it "the mouth of hell."

It wasn't actually burning like hell or like the Darvaza Crater, nicknamed "Door to Hell," in the desert of north Turkmenistan.

The crater, which is 69 meters (226 feet) wide and 30 meters (100 feet) deep, is located in a natural gas field in Ahal Province in Turkmenistan, which has the sixth-largest reserves in the world. The crater's origin is disputed, but the theory most widely accepted involves a Soviet expedition to explore for gas.

A local geologist claims the borehole was set alight in 1971 over fears that it was emitting poisonous gases. It has now been burning for half a century.

Well of Hell is likely a sinkhole

Chris Fogwill, a professor of glaciology and paleoclimatology at Keele University, told the DailyMail that it is most likely a sinkhole caused by the erosion of limestone or moving geological salts or brines. Sinkholes can be caused by any number of activities, including drilling, mining or construction. (Related: Enormous, possibly radioactive sinkhole swallows an acre of Louisiana, causes forced evacuations.)

"The erosion around the edge suggests it is not new," Fogwill noted.

Local authorities say they don't know what lies below. The well is said to be one of the most, if not the most, hated spot in the country and contains the "worst water on the planet."

"It's very deep – we've never reached the bottom of this well, as there's little oxygen and no ventilation," said Salah Babhair, director-general of Mahra's geological survey and mineral resources authority. "We have gone to visit the area and entered the well, reaching more than 50-60 meters down into it. We noticed strange things inside. We also smelled something strange. It's a mysterious situation."

Little can be seen from the edge of the hole, except the birds that fly in and out of its depth. Videographers seeking close-ups of the inside of the well can't capture anything relevant or interesting. Sunlight doesn't extend far into its depth and local superstition has it that objects near the hole can be sucked towards it.

Portal, volcano, home to genies

Some say it is a portal, an extraordinary opening that connects travelers to distant realms. A good portal is a shortcut, a door into the unknown. But experts say that portals open and close dozens of times each day and typically located thousands of miles from Earth.

Some suggest the well is a supervolcano that will eventually erupt but can't back up the theory with scientific evidence. (Related: Volcanoes beneath ice in Antarctica could erupt and leave the planet "uninhabitable," warn researchers.)

Babhair said that the well was "millions and millions" of years old. "These places require more study, research and investigation," he said.

Over the centuries, stories have circulated of malign, supernatural figures known as jinns or genies living in the well. Many local residents remain uneasy about visiting the vast hole, or even talking about it, for fear of ill-fortune.

Yemenis have had enough bad luck as it is. The country has been embroiled in a civil war since 2014 between the government and the Huthi rebels. According to the United Nations, Yemen is suffering the world's worst humanitarian crisis with tens of thousands killed, millions displaced and two-thirds of its nearly 30 million people dependent on some form of aid.

Sources include:

DailyMail.co.uk

TheGuardian.com

NASA.gov