Monitors of Mayhem

For centuries, Monitors have walked among us, studying us. We had nothing they wanted and we were too primitive to interact with them as equals, but from time to time we showed promise.
About to explode in a series of volcanic eruptions, Yellowstone would cover the central plains of the continent with tons of rock. The planet would be darkened for years with ash that would cover the sun and change the climate for at least a decade. Billions of people could starve, and America's international cultural dominance would certainly end.
This was worth observing in greater detail. Even for scientists who were engineering their own genome while humanity's ancestors were still in trees, a planetary-wide cataclysm is a rare opportunity. More Monitors were sent to set up observation centers to see firsthand how human societies would deal with this geologic upheaval.
The Monitors would need the cooperation of their neighbors, if they could be trusted.
"May you live in interesting times" is an ancient Chinese curse whose meaning will soon become abundantly clear.