Deep Underwater Cities
Build cities deep underwater for reducing likelihood of extinction from natural catastrophic events.
Unavoidable natural catastrophies, such as Gamma Ray Bursts could affect life not just on Earth, but potentially on other planets of The Solar System almost simultaneously.
Even right now, as we don't have a colony in Mars, it would make sense building a self-sustaining city deep under the sea. People could live from cultivating sea cultures.
Looks like we've got progress in China: building a sea lab 3km underwater in the South China Sea. Bloomberg writes: "The project was mentioned in China's current five-year economic plan released in March and ranked number two on a list of the top 100 science and technology priorities."