Hundreds of impacts have been reported on Earth. Many happen without being seen by anyone on the ground. On average, asteroids with a diameter of impact Earth about once per year. These usually explode in the upper atmosphere, and most or all of the solids are vaporised. Asteroids with a diameter of collide with Earth every 500,000 years. Large collisions, of , happen once every twenty million years. Some much smaller asteroids have caused damage and injuries, but no human is known to have been killed directly by an impact. The Chelyabinsk meteor event in 2013 is the only impact event known to have resulted in a large number of injuries. It was about 20 metres wide. One of the most well-known impacts in modern times was the Tunguska event. This happened in Siberia in 1908. Impacts of that size are said to happen about once every thousand years.