What is the biggest impact crater on Earth?
The biggest confirmed impact structure is Vredefort in South Africa, formed about 2 billion years ago, with an original diameter estimated at roughly 250–300 km, though erosion has removed the original crater rim.
The largest known impact structure on Earth is Vredefort, south-west of Johannesburg in South Africa. It formed about 2.02 billion years ago, when an asteroid many kilometres across struck what was then a very different-looking planet, with no plants on land and only simple life in the seas.
You might expect a giant hole, but you would not find one. Over two billion years, erosion has stripped away many kilometres of rock, removing the original rim and crater floor. What remains are the deep roots of the structure: a central dome of uplifted ancient rock surrounded by a ring of hills, known as the Vredefort Dome. Geologists estimate the original crater was roughly 250 to 300 km in diameter, although the figure is debated. Evidence of the impact includes shattered rocks called shatter cones, shocked quartz and veins of rock melted by the pressure. The Vredefort Dome was made a UNESCO World Heritage Site in 2005.
Two other giants compete for second place. Sudbury in Ontario, Canada, is about 1.85 billion years old and has been squashed and eroded into an oval basin; it hosts some of the world's richest nickel and copper mines. Chicxulub, buried beneath the Yucatán Peninsula in Mexico, is about 180 km across and only 66 million years old, so it is far better preserved, and it is linked to the extinction of the dinosaurs.
Earth must once have had even larger craters, especially in its first few hundred million years, but plate tectonics, erosion and volcanism have erased them. To see what our planet's early surface looked like, scientists turn to the Moon, where giant basins thousands of kilometres across survive almost untouched.