Yesterday I described lava tubes, and the current evidence for their existence. But why are lava tubes so interesting for engineers investigating the possibility of a manned, long-term lunar base?
The reason is safety - principally with regard to radiation exposure, but other factors come into it as well.
Hazardous radiation in space in the inner solar system divides into two main categories: galactic cosmic rays (GCR) and solar particle events, but for our purposes we can summarize them as high energy particles and low energy particles. On Earth we are protected from the radiation by the Van Allen belts (part of the Earths magnetic field), which trap most of the radiation before it gets to us. We see the interaction between the magnetic field and the particles as the Northern Lights. That's why the Van Allen belts are particularly extensive when there's a solar flare, for instance. But the Moon has no magnetic field, and not even an atmosphere to shield astronauts from radiation. If there was a solar flare that hit the Earth-Moon system, any astronauts on the Moons surface would most likely receive a lethal radiation dose.
Using data from samples brought back from the Moon on the Apollo missions, detailed studies have shown that placing a base in a lava tube with 6 metres of lunar rock and regolith above it would filter out almost all the high energy particles. Less than a metre's depth would shield the base from the effects of a solar flare (De Angelis et al. )
So putting the base inside a lava tube would mean that the lunar base would be cheaper to set up, because it would not be necessary to take heavy and bulky radiation shielding to the moon. It would also mean that astronauts on the Moon would be able to continue to work during a solar flare, instead of cowering inside a hardened radiation shelter (although they couldn't go outside, of course).
 De Angelis, J.W.Wilson, M.S.Clowdsley, J.E.Nealy, D.Humes and J.M.Clem, 'Lunar Lava Tube Radiation Safety Analysis' http://lowdose.tricity.wsu.edu/2001mtg/abstracts/deangelis2.htm