A massive Icelandic volcano is set to erupt, experts in New Zealand and around the world have warned.
Grímsvötn last blew its top in 2011, blasting magma through the Vatnajökull ice cap and forcing the closure of airspace across Europe.
"The lava melts the ice, it flashes into steam," Ronni Grapenthin, a geophysicist at the University of Alaska, told GlacierHub.
"There is a tremendous amount of energy being released in split seconds."
Grímsvötn erupts every five to 10 years. It's been nine years since its last, in 2011, which was its biggest in 100 years. The ash cloud rose 20km, and it briefly threatened to be bigger than the infamous 2010 eruption of nearby Eyjafjallajökull.
The signs are there for an imminent eruption. The Icelandic Meteorological Office (IMO) has been monitoring the site, saying it first started murmuring in January.
"The current conditions of Grímsvötn volcano are such that the water level is rather high and the pressure in the magma chamber below the caldera has reached values comparable to those prior to the last eruption."
Gas measurements showing high concentrations of sulphur dioxide, carbon dioxide and hydrogen suggest the magma is close to the surface.
"It's a lot like opening a soda bottle. When you release the pressure, the gases come out," said Terry Plank, Columbia University volcanologist.
The caldera is located beneath the Vatnajökull ice cap. When it heats up the ice melts, forming a subsurface lake. When this lake floods, it can trigger an eruption.
IMO geoscientists Benedikt Ofeigsson told GlacierHub conditions were "very similar" to those found before the 2011 blast.
"It's showing all the signs of an eruption well within the next month or year."
But will it be a "big one"? The next eruption isn't expected to be too bad, scientists said, noting it only has a big one every 150 to 200 years - and 2011 qualified as a big one.
"Each volcano is different and they behave differently, and you can have different behavior from one eruption to the other," said Sigrun Hreinsdottir of GNS Science in New Zealand.
With the volcano completely under ice, it's hard to tell however.
