How long would the Earth last if the sun became a black hole?

[Radar]

Yeah, this is my thought too. I think any proposal that "a small population could survive off geothermal", while technically true, isn't realistic unless there was a huge advance warning. The sun isn't just "a" heat source: it's THE heat source for our planet. If the sun went out suddenly, the surface would quickly become outer-space levels of dangerous. We're not talking about "oh we'll have to supplement with nuclear or geothermal" -- we're talking about everything that isn't meters and meters belowground needing to be its own self-contained life support.

Frostpunk!

That being said, active mains are not that far away from being viable habitats. Repurposing them would not be that difficult if it was suddenly needed. But... whatever we would manage to scrap together would be a temporary solution. With the whole economy and transport network broken, it would not be possible to maintain the technology long-term. Let alone improve it.