In March 2017, the German Bundestag passed a law concerning the selection of a site for a repository for high-level radioactive waste. This has now been followed up by a report, published late September, on 90 potential locations for a repository in Germany. The law itself calls for a repository that will provide protection against the effects of the waste for a period of one million years. The current approach is to place the waste in underground tunnels, which are then sealed using concrete. The first radioactive containers shall be placed in the repository already in 2050. But how can one ensure the sealing to stay safe over such a long period of time?