Fill the mine shaft (I assume it's in an old salt mine or something like that) with concrete. Any civilisation with sufficiently determined archaeologists to go through a hundred meter of concrete to get to your ancient burial chamber either makes sure to check for radiation (from radon emissions from the concrete, if nothing else) or is too dumb to live anyway.

- Jake

If you only spend 20 minutes of the rest of your life on economics, go spend them here.