Ordovician-Silurian extinction event
From Tardis Wiki, the free Doctor Who reference
The Ordovician-Silurian extinction event was a mysterious mass extinction event which marked the transition from the Ordovician to Silurian eras. Earth's life at this time lived in its oceans. Hypotheses on the event's cause include glaciation, a change in carbon dioxide levels, or an extraterrestrial burst of gamma radiation that destroyed the ozone layer.
The event may have actually been caused by gamma radiation from a Tritovore experiment. It may have been caused by the Sycorax launching one of the first-ever invasions of Earth and taking from the planet a resource they needed. (PROSE: Prehistory Repeating Itself [+]Loading...["Prehistory Repeating Itself (feature)"])
