People,
Scroll down a bit and look at the heading Posidonius Rille. The image is blurry (What are they hiding?). And then notice the statement: "The source of this rille is a shallow pit on the northern crater wall (arrow). The rille then meanders along the edge of the crater floor for over 60 miles (100 km). Finally, it reaches a hole in the crater rim and vanishes into Mare Serenitatis." I would conjecture that it has an underground beginning and an underground ending.
http://volcano.oregonstate.edu/sinuous-rilles
I just Loooove that Never A Straight Answer! (NASA)
Cheers!