Caloris Basin
The Caloris Basin, also called Caloris Planitia, is an impact crater, on Mercury, which is ~1350km in diameter. Caloris is Latin for heat and the basin is so-named because it is near the subsolar point when Mercury is at perihelion. The crater is surrounded by a ring of mountains, ~2km in height.
Caloris was likely created by the impact of an object 100+km in diameter; this impact is believed to have pentrated the crust. It is generally believed that energy from the impact was focused, as with a lens, such that it traveled through the planet and created ~500,000km2 of disrupted terrain on the opposite side. This area is convered with hills, some of which are 1800m in height.
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