You couldn't really place a volcano in a more geopolitically sensitive location than this one: straddling the border of China and North Korea! Whilst not particularly active at the moment, the volcano, named Changbai (in Chinese) or Paektu (in Korean), produced a colossal eruption around 1000 years ago, emitting roughly three times the magma that Krakatoa did during its infamous 1883 eruption. Chainbai/Paektu again erupted in 1903, although much less impressively. It is generally agreed that Chainbai/Paektu is now dormant although, in 2017, it was named as one of the world's most potentially deadly volcanoes given that if the crater lake at its summit (see below) were breached, it would threaten 100,000s nearby.
Mount Changbai (Chinese) or Paektu (Korean) (source).
The location of Changbai / Paektu at the Chinese-North Korean border.
OK, so we know all this and apart from its unfortunate location, why is it weird? Well, for anyone with a basic tectonic map of the Earth, they will see that Chainbai/Paektu is nowhere near a plate boundary. This in itself is not unusual as the volcanoes of Hawaii and the Canaries, for example, share this feature but these volcanoes are in the form of volcanic chains, resulting from hotspot activity. Chainbai/Paektu in contrast sits alone and isolated. No one could explain this weird characteristic until fairly recently when US and Chinese researchers were actually able to find evidence of the presence of the subducted Pacific Plate 600 km beneath where Chainbai/Paektu is situated. This was groundbreaking (literally) in that it was the first time such portions of a subducted plate had been found so deep in the Mantle; it could also explain why this isolated volcanic landform developed where it did (see below). Such a finding has additional significance in that it adds weight (no pun intended) to the relatively new theory of the slab-pull mechanism as contributing to plate tectonic movements.
Pacific Plate subduction beneath the Korean Peninsular and the associated upwelling magma making its way through the breaking-up subducting slab (source).
So now that scientists can explain this mystery, let's hope Chainbai/Paektu remains in its dormant state for years to come, thereby averting what could be a particularly explosive geopolitical event.