Asgore fumbled with his papers. He tried, but he had never gotten the hang of managing all this on his own. Ah, yes. There it was. He looked up, at the scientist sitting across from him at the table. Dr. Gaster sat stiff, with his hands folded in his lap.
"One last thing." Asgore began. "Waterfall is having a bit of a trash problem. Useful things wash down from the surface, but a lot of it isn't salvageable, and our own trash only adds to the piles. Could you try to find a better way to handle it, W. D.?"
Asgore tried to use first names for all his employees, but Gaster was making it difficult. All Asgore knew were his initials, W. D., and after all these years he doubted if those stood for anything. Did the man even have a first name? Still, he persevered.
Gaster sat still and didn't say anything. From experience, Asgore knew he was thinking it over to have an answer ready before acknowledging a question had been asked.
"Yes," he finally said, "I think my latest research will apply here. When I am finished, it will be as if the superfluous waste never existed."
"That's an, ah, interesting figure of speech." Asgore chuckled nervously.
Gaster looked offended. "I always mean every word I say."
Soon afterward the construction started. Gaster shut himself in his lab doing who knew what, while a team of workers excavated a new area of the caverns under Waterfall according to his precise instructions. The new area was to have a gentle downward slope, so any trash would take several days to get washed through by the water. That way useful objects could be picked out.
At the end of the slope would be a wide hole, to be blocked off from the water for now and to be filled at a later time.
Gaster never explained the reasons for his instructions, but they always made sense in hindsight, so people rarely questioned him. It took less effort to just do what he asked and see the result than to wrestle the reason out of him.
Gaster's assistant leaned against the machine and looked over the edge. "So this destroys anything you throw in, huh?"
"Yes, retroactively. Please be careful."
"'Retroactively?'"
"Yes. It would not just erase it from the future, like any ordinary method of destruction, but from the past as well. It is very efficient."
"Can you show me?"
"It is hard to demonstrate, but I will try." He took a small bag from his pocket. "As you see, I have a sealed bag of…" he peered at the label. "…'popato chisps.' I will empty it into the machine's field." He held it over the machine and ripped it open.
"Uh, it was empty, dude."
Gaster sighed. "It was, but it used to not have been empty. The bag was full before I opened it, but when the chips entered the machine they stopped having existed."
"Yeah, right. You're just fooling around with an empty bag."
"Even so, be careful around the machine. It does not matter whether you believe it or not."
"Sure. At least that field of yours looks pretty. I've never seen something so dark."
As Gaster finished installing his machine in the hole he resisted the urge to try it. If he threw something in he would forget he had done so, and might do it again, and again, and again. It would be best to never even start.
When he was done setting up, the bottom of the hole was invisible, shrouded in darkness. He removed the blockade around it so the water could flow through.
As the old heaped-up trash went down the hole, and Waterfall grew less and less cluttered, people gradually forgot that there had been a trash problem in the first place. The amount of garbage that had existed dwindled, people stopped complaining, and their previous complaints vanished from the Royal Archive.
Soon, the matter had never been brought up by Asgore, and Gaster had never heard about it. Gaster had never tried to solve it, and he had never built his machine. The machine in Waterfall vanished, delicate part by delicate part. Only the cavern and the machine's field were left. They couldn't be erased, because that would also erase the reason for their own erasing.
All that was, of course, intentional. Gaster was a busy man. He had found a way to retroactively make room in his past schedule for other work, and seized the opportunity.
But although the causes of the Waterfall garbage disposal disappeared, its effects didn't. A series of curious inconsistencies in the amount of water flowing through Waterfall was brought to the attention of none other than the Royal Scientist, W. D. Gaster. Having lost all memory of his own creation, he was curious and decided to investigate.
The water's flow brought him to the deepest part of Waterfall, the dump. He leaned against a pile of trash and looked over the edge of the ravine. It got dark very quickly. He couldn't see the bottom.
He shifted his weight to get a better look, and with a sudden shift, the pile gave way. It went tumbling down the hole. Gaster tried to regain his balance but fell over. He went over the edge, plunged down, and vanished.
A year later.
Asgore wondered why there hadn't been a Royal Scientist for so long. He remembered losing the last Royal Scientist, half a century ago, but he didn't remember why he never hired a replacement. Had he just never gotten around to it?
Oh well. He made a note to start interviewing applicants.