So, I did some measurements of my backyard. assuming I build relatively close to the house for easy access, I can afford a 22 m x 10 m x 5 m amount of land. With roughly 0.6 meter thick reinforced ...

Hundreds of meters underground. Large enough to house tens to hundreds of thousands of people. And walls strong enough to withstand the force of a nuclear blast. These are some of the world's deepest, ...