Seen here are the spectacular lava-carved rilles called Rimae Prinz.
The rilles of Rimae Prinz next to Prinz crater, captured by NASA's Apollo 15. Image is about 130 km across vertically. Source: Wikipedia
The sinuous rilles of Rimae Prinz were formed during the time of active volcanism on the Moon. The hot lava melted and eroded the surface, leading to the carved rilles. These rilles run for as long as 80 km.
Like the previously covered Rimae Posidonious, the lava must be highly fluid (having low viscosity) to cause such striking twists and turns in the carved channels. Look closely and you see a number of sharp, near 90 degree turns that these rilles take (also seen in the nearby Krieger). How are they caused?
Physically examining and sampling such sinuous and orthogonal rilles via future lunar missions is exciting because they expose layers of past lava flows and underlying structure, giving us an entry point into the volcanic history of the Moon.