Cover image: Artist's rendition of an asteroid hitting Earth. Evidence shows that the Chicxulub crater in Mexico, often cited as proof that a single asteroid wiped out dinosaurs at the Cretaceous-Tertiary (K-T) boundary, was actually created 300,000 years earlier. The authors hypothesize that a complex series of events, including an additional asteroid impact, volcanism, and greenhouse warming, may have precipitated the mass extinction. See the article by Keller et al. on pages 3753–3758. Orbital View of K-T Impact by Don Davis.