So the situation, for those who aren’t familiar, is that Earth is in trouble. Again. A gamma ray burst from a dying star somewhere close by (a couple hundred to a couple thousand ly) has grazed or even struck our planet head on, dumping its payload of high-energy radiation directly into our atmosphere. At this distance, the burst fries half the planet outright, scorching or even glassing whichever hemisphere is facing the burst. Half the global population is dead, the global ecosystem can safely be assumed to be crippled, and so much of the burst’s radiation has attenuated in the atmosphere that our ozone layer has lost all its protective benefits. Anything that stands in direct sunlight after this event is going to be badly burned, meaning the other half of the population and whatever’s left of Earth’s biosphere is living on borrowed time.
Can humans build underground (or underwater) shelters with enough food and water to sustain life beneath the surface of the Earth indefinitely before what little remains of our planet and our civilization collapses, or would these subterranean cities have to either be built prior to the burst or dug out after the fact by the survivors who’d already taken shelter underground before the burst hit? Could humans reasonably be expected to survive this ultimate global apocalypse with our technological know-how and scientific expertise, or is a continued existence underground post-gamma ray burst a fantasy?