. "severity rating" . "Mass Extinction" . . . . . "major extinction events" . "extinctions" . "6th Great Extinction" . "mass extinction of species" . . "cataclysms" . . "catastrophic comet impact" . "van der Hammen cycles" . "major extinctions" . "Extinction event" . "extinguish" . "mass extinctions on Earth" . "major mass extinctions" . "extinction" . . "Mass extinction cycles" . . "threaten species with extinction" . . "Timeline of extinctions" . . "Extinction-Level Event" . . . . "mass-extinctions" . . "Extinction Level Event" . . "largest mass extinctions" . "five previous mass extinction events" . "Mass extinctions" . "vast extinctions" . "big five" . . "extinct" . . . "biotic crises" . "Extinction event" . . "cycle of mass extinctions" . "Extinction" . "threatens the survival of humanity" . "extinction level events" . . . "massive extinction event" . . "mass extinction event" . . "threats to human existence" . "mass extinctions" . "extinction event" . . . . . "The Big Five" . "meteor strike" . . "major extinction event" . . "mass extinction" . . . "past great extinctions" . . . "mass extinction events" . "extinction-level event" . "cataclysm" . "explosion" . "extinction period" .