What it was
Low hills curve around the roots of a missing crater. The Vredefort asteroid has gone, and so has much of the hole it made. What you can walk on in South Africa is the deep rock left behind.
The blow crushed the crust and melted rock. Then the deep floor sprang upward in a central uplift, a raised patch in the middle. Rain and rivers later wore away the rim and much of the floor. The hills are the scar's exposed roots.
When and where
The impact struck near what is now Vredefort about 2.023 billion years ago. A vast thickness of rock has worn away since. As the upper layers vanished, they exposed the deeper damage while making the original crater harder to spot from the ground.
How big
Models suggest a rock 12 to 16 miles across (20 to 25 kilometers), more than twice Everest's height. The crater stretched roughly 155 miles (250 kilometers). Both estimates come from working backward from the remains. Nobody measured the asteroid on its way down.
How we know
Small rocks give the crash away. Shatter cones have grooves made by shock waves. Quartz grains also keep marks of extreme pressure. Along with the bent layers, those clues show that a blow from space damaged this ground. A worn hill alone would not tell us that.
Still a mystery
Scientists still have to work out the exact size and speed of the falling rock. So much of the target has vanished. The gold story is clearer: gold-bearing layers were already there, and the crash bent and buried some of them. Later weather exposed parts again.