These ten ancient events sound like sci-fi plot devices, but each is grounded in scientific evidence. [1]9 Snowball EarthHow Volcanoes Froze the Earth (Twice)Watch this video on YouTubeDuring the Cryogenian Period, Earth experienced two extraordinary deep freezes, the Sturtian and Marinoan glaciations. As more bright ice covered the darker land and ocean, Earth reflected increasing amounts of sunlight back into space. [2]8 When Oxygen Became PoisonHow a single-celled organism almost wiped out life on Earth – Anusuya WillisWatch this video on YouTubeToday, oxygen sustains most complex life on Earth. [6]4 Earth Was Once Inhabited by Giant InsectsWhat Was Earth Like in the Age of Giant Insects?