lukan parent
Because the ground is not static. And we are just starting to understand what is going on down there. So yes, there are sites that remained quite unchanged (like with the natural fission reactor), but personally I remain sceptical with such statements.
Half a kilometer isn't particularly deep. There are dozens of mines over 2 KM deep: https://en.wikipedia.org/wiki/List_of_deepest_mines
Are we supposed to hold off on developing the only geographically independent and non-intermittent form of clean energy because of some vague nebulous fear that waste buried half a kilometer deep in bedrock will come back up to the surface and harm people... somehow?