BOOM!

100 years of Tunguska.

The background is interesting, but it's sort of obviously a spaceship exploding. A blast that large, and we didn't know about it for 20 years? If the ship had landed anywhere else on Earth, we would have (if it had landed in the water, it would have created a civilization-destroying Tsunami). So not only did aliens* visit Earth, but they knew enough about our civilizations to know where to crash land.

What else explains the effect of a meteor crash in the remotest land mass on Earth, without an impact crater or meteor remains?

*Please note that it might not be aliens, but could also be time-travelling humans from the future doing grad school research.