In the timeframe of the geologically recent history of the Earth, say, 100 million years, several large meteorites have hit Earth. The Cretaceous-Tertiary asteroid, for example, is theorized to have caused the extinction of the dinosaurs. If such an object struck Earth it could have a serious impact on civilization. It is even possible that humanity would be completely destroyed; for this, the asteroid would need to be at least 1 km (0.6 miles) in diameter, but probably between 3–10 km (2–6 miles).[10] Asteroids with a 1 km diameter impact the Earth every 500,000 years[10] on average. Larger asteroids are less common. The last large (>10 km) impact happened 65 million years ago. So-called Near-Earth asteroids are regularly being observed.