Maybe it's a sign of a future time travelling civilization with nuclear power but poor navigation, warped straight into the mantle Earth's crust :D

I thought where you were going with his was "that realized the best way to dispose of their nuclear waste was to dump it in the deep past." I’d read that novel.

Only to mine it later and re-use it over and over again. The 5 billion year long recycling program.

A civilization (even perhaps extraterrestrial) that possessed nuclear energy? Unlikely, but still fun to think about! ;-)