The decimation of an entire planet would certainly fall under the category of unhappy memory.

"I've wondered, because I know that in the future of my world, the Earth must be abandoned. Some of the people of the Underside colonize the planets along with those of the Above. But I often wonder who gets left, and what it will be like for them. I mean to say, would there even be a noticeable difference if you never went up again?"