In New Scientist there is an exciting article, Found: first 'skylight' on the moon. The article describes the finding of a deep hole which is a skylight of an intact lunar lava tube. Many partially collapsed lunar lava tubes have been identified this is the first intact lunar lava tube.
This is wonderful news. A lunar lava tube is big cave which could be used to shelter a lunar base. This lunar lava tube seems to be at least 80 meters deep and 370 meters across.
Lunar lava tubes have roofs tens of meters thick, are hundreds of meters wide and thought to be kilometers long. So they would offer a base a lot of protection from radiation and meteors. As well as lots of room to expand.
Lunar lava tubes are thought to be temperature stable at -20 Celsius which makes operations much easier than a surface base subject to the drastic lunar temperature swings.
Lunar lava tubes might contain significant quantities of lunar ice, which seems to be suggested by recent data. Since lunar lava tubes are temperature stable any water entering the cave would freeze and stay inside the cave. I noticed this phenomena when I toured the lava tubes in northern California in the summer, the lava tube contained significant quantities of ice.
We need to find more intact lava tubes by using gravity, radar and seismic sounding surveys. As they will provide a safe an secure location for human expansion off of Earth. Mars is believed to have even larger lava tubes than the Moon.
For more information - Caves on the Moon? by Paul Spudis in The Once and Future Moon