I'm always kind of watching to see if things Musk works on are applicable on Mars. I'm curious if the things he does always have this in mind. Seems to me, the tunnels thing fits this narrative. New high-tech tunneling technology could be quite useful on Mars. Other things useful on Mars -- solar power, batteries, electric vehicles, propulsively landed reusable rockets, new space suits, satellite networks for communications (the Internet satellites thing), vertically integrated manufacturing, self driving technology and sensor suites, low pressure tube transport, and now...tunnels.