Living at least 6 meters underground? Short stays on the surface without protective clothing that has not been invented yet? In the case of Mars, a trailblazing crew of a dozend earthlings would have to be supplied from the earth, because on Mars there is nothing that would suppurt human life. The energy question: To launch a rocket to Mars the chemical energy needed would be staggering. To "slow down" the rocket upon arrival on Mars would be staggering. To bring back someone from Mars would be staggering again. We are technically not be able to accomplish this. PS: Who would want to live on a planet 6 meters below ground? Most of the time. Where would Elon recruit such "volunteers". From a mental institution, promessing them free snacks on Mars? But there is hope. In February, Elon has decided to go for a "Moon Endevor", basically scrapping his "Mars delusions".