A Russian volcano believed to be once extinct may be "waking up", say volcanologists writing in the Journal of Volcanology and Geothermal Research.

Researchers have pointed to continuing seismic activity below Bolshaya Udina, a volcanic massif in the central part of Kamchatka Peninsula, dating back to 2017. Prior to that, there had been no activity recorded to a similar scale. Indeed, the volcano was thought to be extinct since there have been no recent eruptions and experts believed it was unlikely to erupt again in the future.

Lead author Ivan Koulakov, a geophysicist from Russia's A.A. Trofimuk Institute of Petroleum Geology and Geophysics, believes we should now reclassify its status to active.

His announcement follows the recording of 2,400 or so seismic events between October 2017 and February 2019, including a 4.3 magnitude earthquake (the strongest seismic event registered in the area, ever) under Udina last February. In contrast, between 1999 and September 2017 there were just 100 or so weak seismic events detected. 

Last year, Koulakov and his team set up four temporary seismic monitoring stations to record and then study seismic events between May and July 2018. There were a total of 559.

Later analysis revealed that seismic activity was taking place at depths more than 5 kilometers (3 miles) below the surface in an "elliptical cluster" oriented in the north-northeast–south-southwest direction. This – the study authors say – could hint at the presence of magma intrusion with high fluid content. Justification, they add, that could be used to update the volcanoes status from extinct to active. 

But that's not all. These events seem to link the volcano to the Tolud zone, south of the volcano, which appears to hold magma in the Earth's lower crust.

In the Tould zone, continuous seismicity has been reported at depths of 10 to 20 kilometers (6.2 to 12.5 miles) below the surface. The researchers believe seismic activity in this region led to the creation of a new pathway between the zone and Bolshaya Udina sometime in 2018. This is "feeding" the volcano with magma. 

According to Koulakov, there is a 50 percent likelihood of it erupting. Alternatively, "it could just release the energy smoothly over a few months, or it may just disappear without any eruption," he told CNN. 

What kind of threat this poses is unknown. There is only one way to make sure – and that involves tracking seismic activity. "We need to deploy more stations to understand if it's dangerous or not," Koulakov added.