It seems like people are into MASS EXTINCTIONS these days and I wrote a book on them so here's a 2-Part ⚡️MEGATHREAD⚡️ on the worst things that have ever happened 
EXTINCTION 1: The first major mass extinction was 445 million years ago, the End-Ordovician. It happened on a planet that as alien as any in science fiction.
N. America was mostly south of the equator and on its side, eastern New England had just rifted off of a supercontinent straddling the South Pole--and wouldn't crash into N. America for almost 100 million years. The midwest was a shallow ocean.
Pangaea was still more than 100 million years in the future, dinosaurs 200 million. Homo Sapiens 445 million. There was virtually no life on land. Sea life dominated by invertebrates like trilobites (vaguely horseshoe crab-looking things) & squid-like things dangling from shells.
But the Ordovician ended in a punishing ice age that dropped sea level hundreds of feet, upended the circulation and oxygenation of the ocean, destroyed habitat globally and killed most life on earth.
(1 reason we know about this ice age is b/c same kind of rocks scraped up by glaciers from our EXTREMELY RECENT ice age only few thousand yrs ago in N. America, you can also find in the middle of the Sahara--only from 445 MILLION yrs ago, when Africa was over the South Pole.)
This catastrophic ice age was likely caused by falling CO2 (just as more CO2=global warming, less CO2=global cooling).
This CO2 drawdown might have been caused by the erosion of the nascent--and then towering--Appalachian mountains in the tropics, the evolution of tiny land plants, and even a transition to bigger plankton in the oceans, which would have sunk more carbon to the seafloor.
There might have even been an episode of explosive volcanic eruptions at the very end of the Ordovician as well, which caused an "albedo catastrophe" reflecting sunlight back to space and tipped the planet into this ice age.
This brief ice age ended when the climate dramatically reversed a million years later, rebounding into a devastating greenhouse climate with spreading anoxic seas (interestingly, leaving behind a lot of oil in North Africa)--the coup de grace.
So that's the first one.
EXTINCTION 2: The second major mass extinction was perhaps the most mysterious of the "Big Five": the Late Devonian mass extinction.
In the wake of the End-Ordovician, fish--which had previously been unimportant players--took over the seas, and by the Devonian period the world was enjoying what is known as the Age of Fish. Some fish were even beginning to tentatively test out life on land (our ancestors)
But in the ocean, the most dominant fish were an extremely strange group called placoderms. They were heavily armored and some of them, like Cleveland's own Dunkleosteus (pictured), were absolutely terrifying.
A series of mass extinctions, including an extreme catastrophe 375 mil years ago, punished them, and by the end of the Devonian they were all dead. The largest reefs in the history of the planet were also completely destroyed, along with the majority of invertebrate life as well.
So what happened? Trees just evolved on land & spread over the continents--a geoengineering project that, downstream, likely polluted the seas w nutrients, spurring huge algae blooms that died & robbed oceans of oxygen (same thing happening today in dead zones like Gulf of Mexico
And there was another round of mountain building in the Appalachians that, along with the huge new carbon sink of forests, might have contributed to weathering and CO2 drawdown that caused an ice age at the very end of the period.
BUT during the brief, massive extinction pulse itself, 375 million years ago, there's also evidence for an extreme WARMING spike as well (seen in oxygen isotopes from teeth of strange little sea creatures) and sea level RISE.
Which brings us to THE MOST DEADLY FORCE IN EARTH HISTORY: LARGE IGNEOUS PROVINCES:
A huge expanse of volcanic rocks in Russia, called the Viluy Traps, has been dated to sometime near this extinction (traces of mercury in the rocks similarly point to an episode of massive volcanism).
CO2 from these volcanoes would have caused extreme warming (as CO2 does), ocean anoxia and sea level rise, & might have been the main kill mechanism in this mass extinction. (Interestingly, all that dead Devonian sea life makes up much of the natural gas we frack in N. America).
But the Devonian was a weird one and things get less complicated from here on out.
EXTINCTION 3: Fast forward almost another 100 million years to 252 million years ago, and now we get to the worst thing that has ever happened in earth history, the End-Permian mass extinction--PLANET EARTH'S ALL-IS-LOST-MOMENT.
The End-Permian mass extinction will make the much later, much more famous extinction that would wipe out the dinosaurs look like a joke.
By now we're in the golden age of Pangaea: there are strange animals lumbering around on land (not dinosaurs), forests are established and reefs have recovered from the Devonian apocalypse. But after the End-Permian extinction, there wouldn't be much left of anything. END PART 1
