Mass extinction

At least 5 times all life on earth has been on the verge of extinction. Some say that we are close to the sixth. In this episode the scientists take a closer look at the mass extinctions in our history. What did really happen 250 million years ago, when 95 percent of all sea-living species died out? Is there something we can learn, that we can use today, when the number of species on earth is dwindling and the eco systems are quickly changing?