If there is a nuclear war, an asteroid or some other major event or perhaps in case you are running out of space you can build cities underground.
This amazing structure which is the opposite of a skycraper allows people to work and live underground.
This amazing network of tunnels is in Turkey and allowed ancient people to hide in them.
This abandoned railways has been converted into a lush underground park. Here is an underground cave that has been found to have a jungle in Vietnam,
If there is a large asteroid, nuclear war or if we just run out of space we can build underground cities that people can live in.