A volcano once considered extinct has woken up, and scientists fear it could erupt catastrophically at any moment.
It's not known how long ago Bolshaya Udina, in Russia's far east, last blew its top.
"When a volcano is silent for a long time, its first explosion can be catastrophic," seismologist Ivan Koulakov of Russia's AA Trofimuk Institute of Petroleum Geology and Geophysics said, according to Russian news site RT.
An anomalous series of seismic events reportedly began beneath Bolshaya Udina in 2017, and scientists now think it should be reclassified from extinct to active.
"Based on the results of this study, we conclude that during 2018, the Tolud magma source appeared to have built another pathway to Bolshaya Udina."
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Koulakov told CNN there was about a 50 percent chance the 2923m-high Bolshaya Udina would erupt soon.
"Or it could just release the energy smoothly over a few months, or it may just disappear without any eruption."
While few people live near Bolshaya Udina, which is located on northeastern Russia's Kamchatka peninsula, Koulakov says there's a risk to the entire globe.
"A large amount of ash is thrown into the air, it is carried far away, and not only the surrounding settlements, but also large territories all over the planet, can suffer."
And for the villages nearby?
"Recall Pompeii," he said ominously, RT reports. "The awakening of Vesuvius was preceded by a lull for several thousand years."