You might be thinking of the natural gas stimulation experiments performed by the US in the 70s[1]. From that article, "it was found that the three blast caviti

You might be thinking of the natural gas stimulation experiments performed by the US in the 70s[1]. From that article, "it was found that the three blast cavities had not connected as hoped, and the resulting gas still contained unacceptable levels of radionuclides" so it never went past the experimentation phase. [1] https://en.wikipedia.org/wiki/Operation_Plowshare#Natural_gas_stimulation_experiment https://en.wikipedia.org/wiki/Operation_Plowshare#Natural_ga...