We have evidence of a natural nuclear reactor from a billion years ago. Back then, there was a higher concentration of U235, and a uranium deposit with an under

We have evidence of a natural nuclear reactor from a billion years ago. Back then, there was a higher concentration of U235, and a uranium deposit with an underground river actually underwent fission. We can tell because the U235 is gone, and in its place are the daughter nuclides of the fission products. https://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor https://en.wikipedia.org/wiki/Natural_nuclear_fission_reacto... So it seems likely that our nuclear waste deposits will have isotopic concentrations that indicate fission took place. But this time, the U235 concentration is too low for it to have happened naturally.