I blame the humanoid reptiles that live underground they are always digging around and they must have hit the drain pipe for the pond...
Blamed on a nearby quarry. My guess is that the quarry drained off groundwater which removed support for what had been the bottom of the fish pond, and it collapsed. The fish will probably reappear at the quarry...
Terries and Fermies, I tell you!
A favorite edition from my early school days.
https://en.wikipedia.org/wiki/Land_Beneath_the_Ground!
Scrooge is worried about earthquakes damaging his money bin and is determined to find out what causes them. So Scrooge McDuck and his nephews Donald Duck, Huey, Dewey and Louie go underground. As they approach a huge underground cavern, one of the nephews is surprised to see so much light underground and exclaims, “The sun room of the mad march hare!” The ducks meet two types of underground beings known as Terries and Fermies that live in enormous caverns underneath the surface of the earth.