In fact, Alvarez et al. saw this spike in sedimentary rocks from three different locations: Italy, New Zealand, and Denmark. This proves that their iridium spike was not caused by any local phenomenon, but something truly global.

More recent work in fact has found iridium deposits at the Cretaceous-Tertiary boundary all over the world.

http://www.carleton.ca/~tpatters/teaching/intro/extinction/extinction11.html

This caused Alvarez et al. to suggest that an extraterrestial body -- comet or asteroid -- had impacted the Earth 65 million years ago and caused sufficient climatological destruction to wipe out the dinosaurs (and many other creatures).

It's important to realize what a novel idea this one. At the time, no one considered that an impact could have killed the dinosaurs -- this was totally a revolutionary idea.

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