How about the bottom of Mariana tench? The intense pressure will make sure some Mr Rando can’t just pop in one day and smack it with a hammer. If you keep this relic in the remains of the exploded reactor in Chernobyl, some nut job can just run in, cause some damage and run away. Sure, they will pay with their life, but that won’t fix the hammer marks on the cube.

Chemical dangers are another option, but those kinds of places aren’t stable for a million years. Some volcanoes spew sulfur dioxide, which would be a good repellant, but those vents open and close in unexpected ways.