> When it's full, the Finland plan is to fill in everything, seal it off, and not mark it. If some future society can drill through that much hard rock, they probably know about radiation, since that's a routine hazard in mining.

Reminds me of that Star Trek episode where Data, confused, walked with an suitcase with irradiated materials to a village that was just discovering the basic elements. They got irradiated because they couldn't understand the radiation symbols on the suitcase.