Killing comet rain
As we know, about 50,000 AU from the Sun, there is Cloud of Oort, which contains millions of comets rotating around the Sun. They all don't rotate in ecliptic orbit in just one plane like the planets, but all are 'situated' in spherical shape around the Sun.
They are not harmful to the Earth unless and until they kicked by gravitation field of any star passing from near.
If this happens any time, lots of comets starts 'falling' towards the Sun. And they can also fall on the Earth passing on the path of them.
Before 24.7 million years, 96% life on the Earth was destroyed due to it.
Second 'doom' was happened before 20.5 crore years and third, before 13.7 crore years.