Around 2 billion years ago, an asteroid crashed near present-day Johannesburg to carve out what is now the world's largest known crater. The impact initially put a dent in Earth's surface that spanned 60 miles wide, but the crater grew up to three times wider as its walls collapsed and the rocks below began to rebound. Scientists say the impact was larger and more energetic than the one that killed the dinosaurs.
Today, the Vredefort crater looks nothing like its bowl shape during its formation and certainly not the site of a catastrophic global event. Heavy erosion has left only a portion of its ring still discernible. Parts of the crater even include farmland. Yet even finding that amount of evidence from this crater is lucky, scientists say.