Impact craters are like bomb craters. Indeed, we can compare the energy of an impact with that of nuclear bombs. Even relatively small impacts are as strong as a nuclear bomb. The Chicxulub impact that killed the dinosaurs was more than 1000 times stronger than the power of all nuclear bombs together!

Scientists have calculated that asteroids of that size fall on the Earth every 50 to 100 million years. If that would happen today, it would certainly kill most people on Earth.
But even a 500-m-wide rock would cause worldwide changes in climate. If it plunged into ocean, the giant waves (up to 60 metres high) would devastate coastal regions worldwide. Such meteorites strike the Earth more often: every few 100 thousand years or so.

Read about impacts hitting humans: