For (1), we choose well hidden geological location, e.g. stuff that is about a mile deep. My guess is we don't have such an extensive knowledge of geologically

For (1), we choose well hidden geological location, e.g. stuff that is about a mile deep. My guess is we don't have such an extensive knowledge of geologically stable locations one mile below surface. For (2), I'd venture to say that this is based on what we know today. But then, nuclear reaction or coherent light or traveling to the moon were not exactly "plausible" in 1900 either. What we know from physics is that there's still a lot of residual energy that could theoretically be extracted, to the point where these isotopes are naturally radioactive. So to me, that means we are likely to one day figure out how to harness that residual energy.