Gold Reef City is a theme park and casino built on the Crown Mines site, one of the original gold mines that made Johannesburg. It sits directly next to the Apartheid Museum, which makes a combined visit logical — though the juxtaposition of the two experiences (one of Africa's most serious history museums alongside waterslides and slot machines) is jarring. The casino complex is not worth your time.
The mine tour is a different matter. The shaft descends 220 metres underground, and guides walk you through the actual workings — the gold-bearing reef is visible in the rock walls, the scale of the tunnels is striking, and the explanation of the extraction process is genuinely interesting. This is where Johannesburg came from: a reef of gold-bearing conglomerate running 400 kilometres along a ridge that sparked one of history's most intense mining rushes. The tour makes that tangible in a way that no museum exhibit quite manages.