Of all the species that have ever existed on Earth, 99% of them are gone. Most of those were wiped out during what are known as "Mass Extinction Events".
Since life emerged on our planet, there have been five mass extinctions. Sometimes these events last many years. But sometimes a mass extinction can happen very, very suddenly.
Some scientists think we're on the verge of a sixth mass extinction. Others think it's already begun.
But the question everybody asks is: "Is there anything we can do to stop it?"