Sci-fi earth being destroyed by weapon revolving in core. Read before 1995?

[JAMES]

The name was something like Thors Hammer or Hammer of the Gods, definitely had hammer in the name. This was probably over 30 yrs ago that I read it. Some alien race had injected some thing or machines into the earth, which were rotating through earths core causing surface destruction . What was that book?

[Jack]

Sounds familiar but I cannot place it:

Larry Niven, Jerry Pournelle wrote Lucifer's Hammer but that is about a comet hitting the Earth

Auther C Clark wrote Hammer of God, again about a meteor or comet hitting Earth

Possibly a Short story?
Thor's Hammer