Extinction: What Happened to the Dinosaurs, Mastodons, and Dodo Birds? With 25 Projects

Have you seen a dodo bird recently? Do you have mastodons playing in your back yard? Not likely—these species are both extinct, which means the entire population has died out. More than 99 percent of all species, or about 5 billion, have gone extinct since life first formed on Earth 4.5 billion years ago. Some of those species went extinct at the same time in an event know as a mass extinction.

What type of event could cause such a massive die off? This is a question that scientists have a...