I've been working late for a while now to finish up a project. In the evenings after everyone has gone, it gets really, really quiet and a bit dark. I'll be working steadily when I hear a distant "Thoom".
A low, echoing, booming sound that you can feel through the floor. Thoom. Like someone is beating a giant steel drum. Thoom. Thoom. Drums. Drums in the dark.
An ominous sound, and unexplained. Sitting there late in the evening in my dark hole listening to the dull repetitive thumping, it reminds me more than a little of Khazad Dum, the Mines of Moria, from The Lord of the Rings.
A great story, one of my favorites. Of course there are people who feel it should have ended differently and saved everyone the trouble.