At one point, physicists thought detonating even one nuclear bomb might set fire to the atmosphere.

This was taken seriously, and disproven before one in fact was detonated, but it's not clear that the tests wouldn't have gone ahead even if the verdict had come back with merely "unlikely".

In the current day biologists, computer scientists and physicists are all working on devices which could be far more dangerous than nuclear weapons. In this case the danger is well known, but no-one high-status enough to succeed is seriously proposing a moratorium on research. To be fair, we've still got some time to go.