The Big One

How big were these ancient impacts?

What happens when one of these things hits? Many ugly possibilities...

Crater -- Big Hole (even if meteor explodes before impact).
Shock Wave -- can level buildings and trees in an area 100x that of the crater.
Debris -- material 10-100x the mass of the meteoroid can be ejected into the atmosphere.
Climate change -- dust particles can settle into the upper atmosphere for years, reducing the Earth's temperature several degrees, enough to significantly affect global climate.
Acid rain -- atmospheric shock may drive nitrogen-oxygen reactions in the atmosphere, which could lead to nitric acid when interacting with water.

Examples:

Tunguska, Siberia (1908) -- impact of a 30-60 meter stony meteorite, exploding before contact (no crater). Released 10-15 megatons of energy (ie an H-bomb).

A picture of the impact site

The K/T Event (or, The Dinosaur Killer)

The most recent mass extinction came between the Earth's Cretaceous and Tertiary Periods, about 65 million years ago. This included the dinosaurs. What happened to cause this mass extinction? Best current theory: impact!
Luis Alvarez (1980) studied clay layers dating from the K/T boundary, and found anomolously high levels of iridium. Iridium is not common on Earth, but is found in meteorites. Iridium was also found throughout the earth, in layers which date to the K/T boundary.

Current thinking has a 10-km meteoroid impacting the Earth 65 million years ago, causing the climate changes that resulted in the mass extinction. A possible site (Chicxulub) has been identified, in the Yucatan, buried under sedimentary rock.