The document discusses rift valleys on Venus and Earth. It notes that the largest rift valley on Earth is the East African Rift, and provides an image showing a rift valley on Venus near Gula Mons and Sif Mons. While rift valleys on Earth are caused by the movement of tectonic plates, the large rift valleys on Venus are thought to be due to more local tectonic activity, as the surface of Venus appears to consist of a single plate.