Located about an hour northwest of central Johannesburg, The Cradle of Humankind is named for the abundance of hominid fossils found in its limestone caverns, some dating back as far as 3.5 million years. Sure, there are no facts to back it up, but we're pretty sure this was where cave people actually first encountered the Monolith from "2001" and learned to use weapons for conquest. Correct us if we're wrong.
Anyway, the most important cave is Sterkfontein, a World Heritage site that in itself is one of the richest sites on Earth for hominid fossils. Other famous caves include Kromdraai and Wondercave, although there are plenty of less renown chambers to be found in this vast subterranean network that was once home to prehistoric man.
Most are visitor-accessible, so check them out individually for exact locations and the price of admission.