It's April, 1945 and you're a high ranking scientist in the Manhattan Project. You also happen to have the ear of President Truman, so your words and actions have a major impact on world outcomes. Everyone has been working hard on the bomb, determined to defeat the Nazis, except that the Nazis are presently being defeated and you're not nearly as concerned about the war in the Pacific. (You know Japan will lose eventually, regardless of the bomb.)

Another scientist points out to you a disturbing concept: It is possible that the forthcoming A-bomb test (scheduled for July) will cause the atmosphere itself to ignite in a self-sustaining fusion reaction that would consume the entire globe. The consequences would presumably be the extinction of the entire human race and most life on Earth. You don't know if this could really happen, so you do some back of the envelope Fermi estimates and you at least can't disprove the possibility. You might think it's a 0.01%, 1%, 10%, 50%, 90% or near-100% chance. [Pick whichever makes the thought experiment most interesting to you.]

How do you react? Do you resign? What would you tell the president in your next conversation with him? Do you advocate that nuclear testing and the Manhattan Project be immediately halted? If it is halted and the war ends without a test being done, do you advocate for an international ban on testing until the question of atmospheric ignition can be resolved? And how would you propose to resolve the question, from a physical and engineering perspective? (Maybe we can test fusion temperatures inside a device like a particle accelerator or tokamac. The tokamac would be invented a few years later, so you could probably conceptualize it. Or maybe an atomic test could be done deep underground or on the moon.)