Iv'e always loved the wonder caves! it's awesome. It's about 60 meter's deep to the bottom and has a bulging roof that towers up into darkness - it's huge inside. The third largest cave chamber in South Africa really something to experience if you haven't!
A guide takes you down a steep 45 degree stairway below the surface that seems like forever and then down in a lift another 22 meter's. There are beautiful rock formations inside that will blow your mind clear and serene. And this is right outside Johannesburg! How's that.