This is a mosaic of many images taken by Mariner 10 .
It found a huge impact feature on the surface of Mercury, which was named the Caloris Basin. Part of the motivation for the name is that the sun is directly over the basin every second time Mercury passes through its perihelion. It is the hottest place on the planet at that passage.
This major impact feature is just north of the planet's equator and is surrounded by circular mountain ridges up to 2 km (6500 ft) high. The diameter of the basin is 1300 km (810 mi).
Just south of the equator on the opposite side of the planet there is a collection of small jumbled hills. Since this feature is directly opposite the Caloris impact point, it is surmised that they are caused by propagating seismic waves from the impact.