"Oh, that's a big one!"

If that was here in Finland, I would of course be scared because you need a lot of explosives to produce a mushroom cloud, and that would probably mean that:

1. our military has been storing a sizable part of its munitions in one place AND
2. one our eastern neighbours has apparently developed a will to rid our army of its munitions.

...which would mean that some country would be planning to attack us soon.

So, I would probably start preparing for a war. Maybe go buy a lot of food, at least?

In some country further from the Russia I would probably assume it's something non-military. Maybe some form of a very large fertilizer storage? It would be kind of less scary, but of course that would mean a decreased farming output for the next season, and that would be shitty.