When Life Gives You Lemons...

A mass extinction event that occurred 252 million years, otherwise known as the Permian-Triassic extinction event, ago may have been, in part, caused by rain that was as acidic as lemon juice. This rain would have killed off plants and organisms around the world, as was caused by sulphur emission from massive volcano eruptions in the Siberian Traps. The rain may have reached a pH of 2, which is as acidic as lemon juice, and would have been sufficient enough to disfigure plants and stop them growing, leading to their extinction. This would in turn led to the extinction of mammals, as the food supply would be dwindling. After the eruptions ended, rain pH levels would have become less acidic within one year.
The BBC and the Natural History Museum have discussed the Permian-Triassic extinction event in more detail. An estimated 93-97% of all species were wiped out in this event, during the three pulses of extinction. There a several other causes of this extinction. The earlier phase was probably due to environmental change caused by the formation of the supercontinent Pangaea. This landmass caused hot and dry conditions to occur worldwide.

Map of Pangaea (courtesy of Geogrify)

The later phase may have been due to a catastrophic impact event, increased volcanism, explosions from the Siberian Traps, release of methane from the sea floor, sea level change, anoxia (lack of oxygen), increasing aridity and a shift in ocean circulation due to climate change. This caused my favourite fossil group to eventually die out - Trilobites!
Trilobites (digressing slightly here, but never mind!) first appeared 521 million years ago, and had over 17,000 species! They lived in the sea, with some burrowing in the mud, some crawling on the seafloor and some swimming in open waters and reefs. Some trilobites were blind, but many had well developed eyes, with some being on stalks so that the ones who buried themselves in mud could just stick their eyes out like periscopes.

Trilobites (courtesy of BBC)