Determining the Structural Stability of Lunar Lava Tubes [pdf]

The structural and observational evidence seems to support the strong possibility that there are large voids (100s of meters or even kilometer-plus long voids) under the moon's surface.

This seems ideal for future moon bases. If there are already large areas under the surface to protect from radiation and they're structurally stable, it seems like a great starting point. At that point a moon base only needs materials to make it air tight, rather than structurally stable, which means it would require getting much less material into orbit. It might even be possible to accomplish that with heat or chemical treatments of the existing moon rock, further reducing the amount of materials needed.

Since it's also possible to produce oxygen from moon rock (http://curator.jsc.nasa.gov/lunar/lnews/lnmar97/oxygen.htm), it seems like this drastically reduces the cost/complexity of a lunar base.