459777-bc44b982-16ad-11e4-85ab-b94874755dbcWhen we think about the concept of mass extinctions, we tend to think of something pretty dramatic. For instance, we now know that the dinosaurs were wiped out by a six-mile-wide asteroid that hit the Earth at thousands of miles per hour. Its impact, according to the new Smithsonian Channel documentary Mass Extinction: Life at the Brink (airing Sunday night), had the force of “a hundred million nuclear bombs,” unleashing tsunamis hundreds of feet in height that hurtled across the ocean “at the speed of a jet.” So yeah, that’s pretty dramatic. And yet many scientists think that today we may be on the verge of another creeping mass extinction — the sixth the planet has seen — even as most people barely notice it happening. More