Colonizing the lunar surface is a common goal that space agencies from all around the world seem to share. On the other hand, there's a challenge involved in keeping the first lunar colonists safe from cosmic radiation, among other things.
One of the ways we can accomplish this with minimal effort is by utilizing lava tubes. These are fundamentally large caves beneath the lunar surface where lava used to flow way back when the Moon was still geologically-active.
Because many of these lava tubes are deep beneath the lunar surface, the ceiling itself should shield inhabitants from hazards like radiation and space rock impacts. Furthermore, these lava tubes should theoretically be large enough to house entire colonies.
We're still a long way from building cities on the Moon, but by studying these seemingly-insignificant sites further, we might be able to uncover more about the challenges involved with long-term settlement on the Moon (or perhaps elsewhere in the solar system).