Mass Extinction -

1. Since beginning many species evolved and got extinct (in fact only 1 % is living).
2. Sometimes it occured suddenly at large scale, (most recently dinosaurs about 60 million years ago) the reason is still a mystry.
3. High concentration of irridium (rare on the earth) in a thin layer below surface soil (60 million years old) is present in meteorites.
4. This suggests that some comet or meteorites hit the earth around that time.
5. The energy equal to several thousand hydrogen bombs would have been the reason for mass extinction.
6. Alternate hypothesis is global cooling.