There'll be issues keeping that cool and the air fresh putting enough people into them. Also some of the most endangered places have huge populations that would make sinking enough shelters difficult. Depending on the soil composition that might be a short term solution too. The soil surrounding the London Underground is particularly bad for this and it's estimated that 80% of the heat ever generated in the Underground is still there in the walls.

https://www.youtube.com/watch?v=hQo6_GkITe0