I wouldn't have wanted to be near it when it was reacting, however!!
The authors of the article postulated that the critical mass was obtained when uranium salts were washed down this ancient stream bed and concentrated in a whirlpool or something.
It attracted the attention of the nuclear community when it was discovered that the U235 content at this location was only 0.4% instead of the more universal 0.7% found in uranium ores worldwide.
They called it evidence that nature had indeed formed a nuclear reactor.
I wouldn't have wanted to be near it when it was reacting, however!!
Who ever said that nature was safe?