evacuation: to empty or remove the contents of a place. Often done in the case of disaster. Fire, flood, earthquake, gas leaks. Sometimes some of all (Japan, we are still sorry that it happened to you.) Just tonight I've had to leave my home because of fire alarms; I was sweating onions on the stove, and though I had the fan on, I thought perhaps I had set off the alarms. Living in an apartment building, I climbed on a chair to turn off the alarm, but it failed. So I moved to the other fire alarm (a new building, they are very thorough) and turned it off as well. But still it blared. So I fled the building in flip flops and coat, and went for a drink with my fiance, and returned rather a bit later. The evacuation had ended.
We still have no idea why it happened. That's the way of it, sometimes.