Iceland’s biggest active volcano is being kept under close surveillance amid signs it is waking up after centuries of slumber.

A new 1km-wide caldera – a basin-shaped volcanic depression – has been discovered by scientists in Öræfajökull, which translates as “wasteland”, in the south of the island.

The Icelandic Met Office has also received reports of the surrounding area smelling of sulphur, while geothermal water has been released from the volcano into a river on the surrounding glacier, reports Iceland Magazine.

Scientists believe this water caused a section of the volcano to collapse, producing the new caldera.

Although scientists say there are no imminent signs of an eruption, Iceland’s Civil Protection Agency has declared an uncertainty phase – a warning that there may be a threat in the near future – while its Met Office has issued a yellow warning.

Bryndís Ýr Gísladóttir, natural resource specialist at the Met Office, told newspaper Morgunbladid: “We issued a yellow warning for security reasons because we actually don’t know that much about Öræfajökull glacier, nor how it behaves because its last eruption occurred in 1727, and 1362 before that.”

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Öræfajökull features Iceland’s highest peak and is thought to be one of the most powerful volcanoes in Europe. It is responsible for the country’s second deadliest eruption after a steam blast in 1362 deposited 10 cubic kilometres of debris across farmland and killed all inhabitants across dozens of farms.

Although still sparsely populated, the region can attract thousands of tourists at the height of the holiday season. The Icelandic Civil Protection Agency estimates there would only be a 20-minute warning before any eruption.

Iceland: Step into the mouth of a volcano Show all 3 1 /3 Iceland: Step into the mouth of a volcano Iceland: Step into the mouth of a volcano 80-volcano1.jpg Inside the Thrihnukagigur volcano Iceland: Step into the mouth of a volcano 80-volcano2.jpg Dressed for the part, going into the Thrihnukagigur volcano Iceland: Step into the mouth of a volcano 80-volcano3.jpg Making the descent

The volcano last erupted in 1727, and as a result volcanologists have a limited ability to predict when any eruption would occur.