What Caused the Lunar Rilles?

I remember reading about the early days of moon exploration…and I was looking at some high-magnification photos of the moon. The lunar rilles are those snakey-river-looking cracks, which look a lot like dry river beds.
Have geologists finally agreed on what they are? I’m sure liquid water never existed on the lunar surface…so, anybody know what the origine of these features was?

The reason they look like snakey river cracks is because that’s exactly what they are. The rilles were once roiling rivers of lava.

The Apollo 15 mission landed near Hadley Rille, and I think some of the most beautiful photos of the moon were taken there.

I believe the scientists have understood the rilles’ origin fairly well for some time. However, the Apollo 15 mission allowed them to examine samples.

This is covered well in Andrew Chaikin’s book “A Man on the Moon.”