One theme that is central to this story's message centers around the importance of caring for the earth and its resources.
When people first discover the hole, they are shocked by its depth. A reporter shows up to investigate, and then a scientist tries to discern the depths of the hole. It is utterly spectacular; no one has ever seen anything like it.
Instead of pausing to appreciate the wonder of such a natural phenomenon, the people quickly begin to use it to absorb manmade waste, grotesque by comparison. The hole first receives nuclear waste, often symbolic of the deadly potential of mankind. Later, boxes of classified documents are tossed in. The decaying bodies of animals which have been decimated through human experimentation are next thrown into the hole.
All of this waste has been generated by mankind, which looks for a quick and convenient means of ridding itself of the byproducts of its efforts. Not having to look at their own waste gives them a "peace of mind," and they concentrate on producing "one thing after another." No one wants to think about the "consequences" of all their production, destruction, and disposal.
Eventually, of course, the people of the town must pay for the way they have disrespected nature, as is evidenced when the workman hears that initial call echoed back to the town in the final lines. The hole is symbolic of Earth itself; across the world, humans stuff our trash into the Earth's hills, plains, oceans, and mountains, trying not to think about the eventual reckoning that mankind will have to pay for disrespecting the natural wonder of our planet. Like the workman, all too often we have our eye on the manmade "skyline," which we are continually trying to improve and grow, and we fail to notice the impending disaster our efforts are creating.