...water filtering down through crevices in the rock played a key role. Without water, it would have been nearly impossible for natural reactors to sustain chain reactions.
The water slowed the subatomic particles or neutrons that were cast out from the uranium so that they could hit-and split-other atoms. Without the water, the neutrons would move so fast that they would just bounce off, like skipping a rock across the water, and not produce nuclear chain reactions. When the heat from the reactions became too great, the water turned to steam and stopped slowing the neutrons. The reactions then slowed until the water cooled. Then the process could begin again.
(sort of via APOD)