To say that space is tight in the Principality of Monaco is an understatement. The pint-sized country, which has a surface area of just 0.78 square miles (2.02 square kilometers), boasts a total population density of 48,466 people per square mile — the highest in the world. With building space stretched to the limit, architects in the world's second-smallest country have been constructing up towards the sky, tunneling down into the ground, and even carving out the surrounding cliffs. Now, they have built a multi-million dollar luxury home inside a rock.