Barks' commentaries:

What is down under the surface of the Earth? Geologists say a strong layer of rocky stuff and, below that, hot soup. Uncle Scrooge got to worrying about how strong that layer of rocky stuff might be, and he set about digging a hole down through it. He never got deep enough to find the hot soup, but he got into some hot soup of another kind.

In this science-fiction adventure Uncle Scrooge and his helpful nephews find the cause of earthquakes, and they unwittingly trigger one that would jolt Richter right off his scale.

I can't think of how I happened to come up with the idea. I suppose I was always figuring on poor old Uncle Scrooge and his problems with his money. I couldn't have the Beagle Boys always being the guys who were trying to bore into it. There had to be other menaces. One menace that came up naturally was an earthquake. It's about the only thing that could affect that tremendous bin with its walls so many feet thick. A cyclone or rain or lightning couldn't hurt it. It must have been with that thought in mind that I got to working on the idea of having Scrooge drill a hole down into the ground to see what was under there, and how solid the foundation was. Would they find a big geyser of molten metal or lava, or would it be something else completely unexpected?
I took the idea that it must be the completely unexpected. They find a hole filled with queer creatures that live down there. I thought of lizards and all sorts of dragons before I came up with these comical little round-bodied guys. After I invented them, I decided they could be the guys who made earthquakes. The story was then built from the problem of how Scrooge could combat an earthquake, moved to his exploring to see how vulnerable he was, then to the guys who actually cause earthquakes.

At the time I wrote the story, the popular stations around San Jacinto were the ones that played western music, cowboy music. That's all I ever listened to on the radio. I didn't have a television, in fact, televisions were not very common in that early time. I would listen to that cowboy music. It became part of my nature to think in terms of a lot of people listening to it. Out here in the West and in Texas, Oklahoma, and Arkansas, western expressions like So Long, pardner and They went that-a-way were just part of the way people thought. So I was thinking the same way, and I had the Terries and the Fermies talk that way because it sounded a little funny. One of them says: You should hear the fellows talk that live under a place called Boston. The ones that lived underneath Duckburg talked this western lingo. The ones from Asia talked Siamese or Hindi or whatever it was that the radio stations over their heads broadcast through the rocks and shale slabs.

I got Donald into the head-butting contest to give him something to do. With Scrooge being the main protagonist and the three kids and their Junior Woodchucks' Guide Book being the brains that always saved him, Donald needed some lines. So he had to be shoved into a situation like that in order to justify paying him any salary. 30 cents an hour, he got!