both round and square have their advantages and disadvantages. Round is incredibly strong, but has less available space, and more importantly, less usable space. Square provides more room, more usable space, and an easier walking platform. The trade off, of course, is less strength.

In a perfect, and high budget world, I would do a combo of both... domed roof/upper walls over a square base with 5' straight pony walls.

The biggest problem with underground, especially up here with our fluxuations in temperature, and more importantly, humidity... is ventilation. Condensation dripping constantly, really fucking sucks!