There is a piece of rock in the Free State of South Africa that looks like a bar of chocolate. Look closely at the Chocolate Tablet Breccia and you see a grid of intersecting cracks, cutting the stone into neat rectangular blocks that really do resemble the segments of a chocolate tablet. It is a small thing, and also a very large one, because these cracks are a record of the shock that hit this land about two billion years ago.
That shock is the Vredefort impact, the biggest known asteroid strike on Earth. The rocks near this site were shattered, melted and shoved upright in the minutes after impact, and what you walk on today is the eroded centre of the structure, now called the Vredefort Dome and inscribed by UNESCO in 2005. Geologists come from all over the world to study it. You do not need any training to enjoy the visit, though a good guide, or at least a local geological map, makes it much richer.
The landscape is rolling farmland and low ridges, with grassland, maize fields and thorn trees, a quiet part of the interior highveld. Roads are mostly good tarmac, with long gentle curves and gravel farm turn-offs, and the surface can get rough near the dome. Traffic is light. Watch for livestock, and for minibus taxis, which stop and start without warning. In summer, afternoons bring thunderstorms; in winter, mornings are cold and clear.
Potchefstroom, a university town of about 120,000 people twenty-odd kilometres away, is where to fill the tank, eat and sleep. It is one of the oldest towns in the old Transvaal, with a good-looking centre and plenty of cafes. At the site itself, expect nothing but rock and a gate, and note that many of the places in the dome are on private farmland, so check access before heading off the main road.