Right now, there are dozens of theoretical proposals for how humans could eventually populate Mars (or the Moon), each as crazy as the next: Space elevator. Inflatables. Giant 3D printer. But we could just go underground. That's the concept behind this plan by Russian architects ZA. Here's the bare bones version: A series of robots—equipped with the ability to dig—would identify weak areas in the Martian soil and carve them out. Then, using a process similar to that of structural 3D printing, these robots would "print" interior structures using the leftover basalt—the most common soil on Mars. "This material is already in use in the aerospace and automotive industries," architect Arina Ageeva told Dezeen. "It is stronger and lighter than steel, easier to operate, fireproof and it does not corrode."