“Well, you have to remember,” Asaro recalls, “that we were amateurs in this field. Walter was a geologist specializing in paleomagnetism, Luis was a physicist and I was a nuclear chemist. And now here we were telling paleontologists that we had solved a problem that had eluded them for over a century. It's not terribly surprising that they didn't embrace it immediately.” As Luis Alvarez joked: “We were caught practicing geology without a license.”

In 1956, Russian astronomer Joseph Shklovsky (1916-85) became the first scientist to consider the extinction was due to a single catastrophic event when he theorized that a supernova (the explosion of a dying star) showered the earth in radiation that could have killed the dinosaurs.

This is a paragraphIn the early 1970s, Walter Alvarez was doing fieldwork in Italy in a comely defile known as the Bottaccione Gorge, near the Umbrian hill town of Gubbio, when he grew curious about a thin band of reddish clay that divided two ancient layers of limestone—one from the Cretaceous period, the other from the Tertiary. This is a point known to geology as the KT boundary and it marks the time, sixty-five million years ago, when the dinosaurs and roughly half the world's other species of animals abruptly vanished from the fossil record. K comes either from the Greek Kreta or German Kreide. Both conveniently mean “chalk,” which is also what Cretaceous means Alvarez wondered what it was about a thin lamina of clay, barely a quarter of an inch thick, that could account for such a dramatic moment in Earth's history. But the thinness of the clay layer clearly suggested that in Umbria, if nowhere else, something rather more abrupt had happened. Unfortunately in the 1970s no tests existed for determining how long such a deposit might have taken to accumulate.

In the normal course of things, Alvarez almost certainly would have had to leave the problem at that, but luckily he had an impeccable connection to someone outside his discipline who could help—his father, Luis. Luis Alvarez was an eminent nuclear physicist; he had won the Nobel Prize for physics the previous decade. He had always been mildly scornful of his son's attachment to rocks, but this problem intrigued him. It occurred to him that the answer might lie in dust from space.

Alvarez knew that a colleague of his at the Lawrence Berkeley Laboratory in California, Frank Asaro, had developed a technique for measuring very precisely the chemical composition of clays using a process called neutron activation analysis. Avarez requested Asaro to test their samples. Asaro recalled in an interview in 2002. “And it seemed an interesting challenge, so I agreed to try. Unfortunately, I had a lot of other work to do, so it was eight months before I could get to it.”

The results were so unexpected, in fact, that the three scientists at first thought they had to be wrong. The amount of iridium in the Alvarez sample was more than three hundred times normal levels—far beyond anything they might have predicted.

Among exotic elements that come from space (some thirty thousand metric tons each year), there is the element IRIDIUM, which is a thousand times more abundant in space than in the Earth's crust. (it is thought, most of the iridium on Earth sank to the core when the planet was young). Over the following months Asaro and his colleague Helen Michel worked up to thirty hours at a stretch (“Once you started you couldn't stop,” Asaro explained) analyzing samples, always with the same results.

Tests on other samples—from Denmark, Spain, France, New Zealand, Antarctica—showed that the iridium deposit was worldwide and greatly elevated everywhere, sometimes by as much as five hundred times normal levels.Clearly something big and abrupt, and probably cataclysmic, had produced this arresting spike.

After much thought, the Alvarezes concluded that the most plausible explanation—plausible to them, at any rate—was that the Earth had been struck by an asteroid or comet. But they had no clue where the impact site was.

The thin layer of iridium was deposited following the impact of a large meteor, comet or asteroid with the earth. Furthermore, this bolide impact (the meteor, comet or asteroid colliding with the earth's surface) could have caused the extinction of the dinosaurs.

At the time, the Alvarez theory was so far removed from prevailing hypotheses that it was ridiculed. There was a belief that terrestrial processes were gradual and had been elemental in natural history. By the 1980s, catastrophism had been out of fashion for so long that it had become literally unthinkable. For most geologists the idea of a devastating impact was, as Eugene Shoemaker noted, “against their scientific religion.” As late as 1988 more than half of all American paleontologists contacted in a survey continued to believe that the extinction of the dinosaurs was in no way related to an asteroid or cometary impact.

By chance in 1990 one of the searchers, Alan Hildebrand of the University of Arizona, met a reporter from the Houston Chronicle who happened to know about a large, unexplained ring formation, 120 miles wide and 30 miles deep, under Mexico's Yucatán Peninsula at Chicxulub, near the city of Progreso, about 600 miles due south of New Orleans. The formation had been found by Pemex, the Mexican oil company, in 1952—the year, coincidentally, that Gene Shoemaker first visited Meteor Crater in Arizona—but the company's geologists had concluded that it was volcanic, in line with the thinking of the day. Hildebrand traveled to the site and decided fairly swiftly that they had their crater. By early 1991 it had been established to nearly everyone's satisfaction that Chicxulub was the impact site.

Still, many people didn't quite grasp what an impact could do. As Stephen Jay Gould recalled in one of his essays: “I remember harboring some strong initial doubts about the efficacy of such an event . . . Why should an object only six miles across wreak such havoc upon a planet with a diameter of eight thousand miles?”

Shoemakers and Levy discovered a comet named Shoemaker-Levy 9, which they soon realized was headed for Jupiter. For the first time, humans would be able to witness a cosmic collision—and witness it very well thanks to the new Hubble space telescope. Most astronomers, according to Curtis Peebles, expected little, particularly as the comet was not a coherent sphere but a string of twenty-one fragments. “My sense,” wrote one, “is that Jupiter will swallow these comets up without so much as a burp.” One week before the impact, Nature ran an article, “The Big Fizzle Is Coming,” predicting that the impact would constitute nothing more than a meteor shower.

The impacts began on July 16, 1994, went on for a week and were bigger by far than anyone—with the possible exception of Gene Shoemaker—expected. One fragment, known as Nucleus G, struck with the force of about six million megatons—seventy-five times more than all the nuclear weaponry in existence. Nucleus G was only about the size of a small mountain, but it created wounds in the Jovian surface the size of Earth.

The K-T extinction was not the first such massive die-off in history, nor was it the largest. The Permian-Triassic extinction event, known as the Great Dying, occurred 251.4 million years ago and eradicated 96 percent of all marine species and 70 percent of all terrestrial vertebrates species on earth

NOTE: This Information has been collected from various books and credible sources. Most of the credit goes to Bill Bryson.