The challenge of subterranean fires

Russian subterranean fires have been burning at roughly the same latitude as Poland, UK and southern Canada; they are mid-latitude organic soil fires, not Arctic.
Further south, in US this year the Duck Lake fire in Michigan and the Muskego muck fire near Bucyrus, Ohio have both been partly subterranean fires.
In the Muskego fire, "The fire was most likely caused by spontaneous combustion. As peat moss decomposes, it heats up. It was so hot and dry that it caught fire, Canterbury said." (Bucyrus Telegram Forum)

Drying out organic soils is a fire hazard.

More from Bucyrus:
"Two other options for extinguishing the fire -- flooding the field or digging the fire up -- are not feasible because of the conditions of the ground and the lack of water sources available.
There isn't a big enough water source close to the area to flood the field, and because the ground is unstable, almost quicksand-like, trucks would be in danger of getting stuck. With ground temperatures anywhere from 200 to 400 degrees, digging up the fire is out of the question as well."

From Newberry, Michigan:
"When underground organic mass such as swamp peat (a heavy turf of decayed vegetation and moss) catches fire, said Chingwa, "it doesn't even look like it's burning. It just looks like dry leaves, but it's extremely hot. And it can move across large areas without you even knowing it.”  Benjamin Reeves, IBT