Lava Tubes
This image shows a chain of small pits and ridge segments. It marks the
likely site of a partially collapsed lava tube in western Mare Imbrium.
Here, instead of a surface lava channel, lava flowed through a buried
tunnel in the mare. After the eruption stopped, the tunnel then emptied.
Where the roof of the tunnel has fallen in, we see pits. If the entire
roof had fallen in, we might see a sinuous trough like other lunar rilles.
Thus, it is believed that some lunar rilles mark collapsed lava tubes.
These rilles are usually near other collapse pits, and some also merge
with ridge-like features. Thus, uncollapsed lava tubes may still exist
near some of these lunar sinuous rilles. (Lunar Orbiter image V-182-M,
from Wilhelms (1987) The Geologic History of the Moon, USGS Prof. Paper
1348.)