Well, consider there are entire buildings built under street level too. I used to work in one. It was an electric power grid command center that was designed to withstand damned near anything that could come its way. It was kind of cool, but not having any windows anywhere and working in the dark made it hard to tell what time of day it was. I always felt like it was nighttime and would be a little put-off when I'd emerge into daylight when the work day was done.