440 Million BC: The Gamma-Ray Burst That Almost Destroyed Earth

Ninety-nine percent of species that ever lived are gone, and this walks through the five mass extinctions that did most of the killing, from a gamma-ray burst that may have stripped the ozone 440 million years ago to the volcanism that nearly ended everything. The question underneath is what made some of them survivable.

Why it's free: Free on Spark, Little Dot Studios' licensed documentary channel.