The Mariner 10 mission to Mercury in the 1970s hinted at long, high cliffs traversing the planet's surface. But that mission imaged just under half of the planetand astronomy's cameras have improved since then.
This image from the MESSENGER flyby shows a region, previously unseen by any spacecraft, that includes a large cliff crossing vertically through the far right.
The presence of many long, high scarps suggests a history for Mercury unlike that of any other planet in the solar system. The giant cliffs probably formed when Mercury's interior cooled and the entire planet shrank as a result.