This is the one I was referring to, though I guess it's just over half a kilometer deep: https://en.m.wikipedia.org/wiki/Onkalo_spent_nuclear_fuel_repository ht

This is the one I was referring to, though I guess it's just over half a kilometer deep: https://en.m.wikipedia.org/wiki/Onkalo_spent_nuclear_fuel_repository https://en.m.wikipedia.org/wiki/Onkalo_spent_nuclear_fuel_re... The Asse II site used an existing mine to avoid having to excavate a new tunnel, which subsequently flooded.