IIRC, this could not happen today, as the concentration of U235 in natural uranium has decreased (by decay) to the point where there's not enough for a chain re

IIRC, this could not happen today, as the concentration of U235 in natural uranium has decreased (by decay) to the point where there's not enough for a chain reaction to be sustained with any naturally-occurring moderator. Some of the mined uranium was found to have a lower concentration of uranium-235 than expected, as if it had already been in a reactor. When geologists investigated they also found products typical of a reactor. They concluded that the deposit had been in a reactor: a natural nuclear fission reactor, around 1.8 to 1.7 billion years BP – in the Paleoproterozoic Era during Precambrian times. At that time the natural uranium had a concentration of about 3% 235U, and could have reached criticality with natural water as neutron moderator allowed by the special geometry of the deposit. It sputtered on and off for hundreds of thousands of years: https://ans.org/pi/np/oklo/ https://ans.org/pi/np/oklo/