At what point will the earth's surface temperature become too hot for humans to survive and force people to move underground?

Google AI's response:

Humans cannot survive a wet-bulb temperature—which accounts for both heat and humidity—of (35C) (95F) for more than a few hours. At this limit, the air is so hot and humid that the body can no longer cool itself through sweating, leading to fatal heatstroke. Instead of moving humanity entirely underground, catastrophic heat will force populations to abandon equatorial regions and relocate to cooler, higher latitudes. While local subterranean infrastructure or climate-controlled habitats will be used for survival in the hottest zones, moving the global population underground is not a viable long-term solution.