Why did nearly all life on Earth die 250 million years ago?

Among paleontologists, it's sometimes called the "Great Dying." Roughly a quarter of a billion years ago, 90-95 percent of all life on Earth died out. It took 30 million years for the planet to recover. What happened?
Most people are familiar with the extinction event 65 million years ago that wiped out the dinosaurs. But the Great Dying was much more devastating. It left almost nothing alive.
Welcome to the Permian
Let's start with the lay of the land. The era before the Great Dying - also known as the Permo-Triassic Extinction - is called the Permian, and it was a time of rapid animal evolution, including mammal-reptile hybrids called synapsids that looked sort of like giant lizards - some even had big sails on their backs. These early mammals roamed a massive landmass called Pangaea, while the one, giant ocean called Panthalassa teemed with sea creatures, from tiny single-celled organisms to trilobytes and large fish. On land, vast forests of giant ferns were giving way to trees similar to the ones we have on Earth today, dropping seeds in order to reproduce.

Basically there was an entire ecosystem of plants and animals on sea and land that you would hardly recognize as earthly - it was as if our planet wasn't really our planet at all. And then a series of catastrophic events managed to destroy most of the life that existed.
The extinction event
Looking at the fossil record, it's clear that there was an abrupt, massive decline in animal diversity.

In this chart, you can see that there were actually three die-offs during the Permian, but the one at the end of the Permian and the beginning of the Triassic, 250 million years ago, was extreme. Writing in the Proceedings of the Royal Society B, Earth scientists Sarda Sahney and Michael J Benton call it "the most devastating ecological event of all time." They explain the chart above, which looks at die-offs of tetrapods (four-legged creatures):

Global diversity (dashed line) and mean alpha diversity (solid line) of Permo-Triassic tetrapod families. Extinctions are labelled as 1, Olson's extinction; 2, end-Guadalupian extinction; and 3, end-Permian extinction.

The seas were hit as hard as the land. Sahney and Benton continue:

The impact of the end-Permian event was devastating. In the sea, the level of species loss was 80–96%, and blastoid echinoderms, tabulate and rugose corals, graptolites, trilobites, eurypterids, acanthodians and placoderms disappeared entirely. On land, the dominant Glossopteris flora was replaced, eight orders of insects became extinct and two-thirds of tetrapod families were lost. The only tetrapod lineages to survive were procolophonoids, dicynodonts, and presumably therocephalians, cynodonts, and archosauromorphs, and their Triassic recovery was slow.

Put another way: It's likely that 9 out of 10 marine species and 7 out of 10 land species went extinct. Moreover, this was the only extinction event on Earth that destroyed many species of insects as well as animals.
When you look at the geologic record, there is simply nothing there at the hinge between Permian and Triassic.

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