Sinuous rilles are probably the most recognizable of small volcanic features on the Moon. Many partially resemble river valleys on the Earth. However, the lunar rilles usually flow away from small pit structures. Also, the lunar samples indicate that the Moon has always been bone dry. Thus, the sinuous rilles probably mark lava channels or collapsed lava tubes that formed during mare volcanism. Still, in some cases, the lunar flows may have melted their way down into older rocks, much like rivers cut into their flood plains on Earth.