"A major fuel spill in Siberia has prompted Russian President Vladimir Putin to declare a state of emergency in an environmentally sensitive Arctic region"

WaPo:

believes the region’s melting permafrost caused a reservoir to collapse, touching off the leak.

...

The current infrastructure present in the Arctic was constructed on the assumption that permafrost would, as its name suggests, be permanent.

...

The station’s employees originally tried to contain the leak on their own and did not report the incident to emergency services for two days