It has been 25 years since my Berkeley colleagues discovered what killed the dinosaurs. What began as a highly controversial interpretation of an experimental measurement has now evolved into the standard explanation of the great extinctions that took place 65 million years ago. And the progress in those 25 years illustrates much about the reality of the scientific method. Two observations: science is not driven by curiosity, but by a sense of adventure; and the scientists role model is not Diogenes, but Sisyphus.

The key discovery was the existence of a thin clay layer near the boundary between rocks of the Cretaceous period (full of dinosaurs) and those of the Tertiary (the age of mammals). Walter Alvarez, a junior member of Berkeleys geology department, noticed that millimeter-sized animals called forams went extinct right at that boundary. He knew the dinosaurs went extinct at about the same time (their disappearance was not well dated), and he speculated that maybe they were killed together. His father Luis Alvarez suggested they measure iridium in the layer, so they solicited help from nuclear chemists Frank Asaro and Helen Michel. The level of iridium they discovered was so high that they were forced to conclude that a comet or asteroid rammed into the Earth, and changed the nature of life.

Was the Alvarez team driven by curiosity? Many say yes, of course they werebut I will convince you that scientists are rarely motivated by it. Curiosity is the impulse to learn something new, and truly curious people spend most of their time reading. In just a few books you can learn a thousand times as much as a top scientist could possibly discover in a lifetime of research.

No, scientists are driven by a sense of adventure, by the desire to be there first, by a hope that for a few days they may be the only people in the world to know some important new fact. Luis Alvarez had scientific heroes, but he also admired and emulated Captain James Cook, who explored the Pacific, and the archeologist Howard Carter, who discovered Tutankhamens golden tomb.

In an adventure, the explorer/scientist doesnt know where he is going, and most of his colleagues probably think he is wasting his time. That was the way I felt about Walter Alvarez 25 years ago, and I wasnt alone. A senior geologist tried to persuade the young assistant professor Alvarez to abandon his ridiculous project, lest it embarrass the department. The quixotic project (he thought) held no prospect for tenure. Christopher Columbus endured similar ridicule. Indeed, the toughest part of innovative research is keeping your own belief in your trek, at a time when people you respect are snickering.

Another danger of scientific exploration is attack by natives: the referees who reject your articles and give poor evaluations to your proposals. Twenty-five years ago I sensed paleontologists complaining, What right does a physicist have to land on our island? Unlike his hero Captain Cook, Alvarez survived these attacks, although to this day there are some aborigines lurking in the jungle hoping to get off a good shot. The Alvarez team survived the tribulations with courage and energy. Want a good image of the Luis Alvarez that I remember? Think of Indiana Jones.

We have learned a lot in the years following their great discovery. The impact crater was located, buried under the Yucatan peninsula of Mexico. But we dont really knowdespite strong arguments on both sideswhether it was an asteroid or a comet that hit. Other iridium layers were found at other ages, but none as overwhelmingly strong as at the Cretaceous-Tertiary boundary. Was the great Permian-Triassic extinction, 250 million years ago, also caused by an impact? Every few years a paper is published claiming that the answer is yes, but the evidence has been weak; in my opinion, the answer is still unknown. Confusing the issue are great outflows of lava that occurred at that same time. Is that an accidental coincidence? Similar flows were present at the time of the Cretaceous extinction. Do we really understand that event as well as we thought?

Another unanswered question: did the Cretaceous bash take place by itself, or as part of a comet showerthat is, a period of intensive bombardment by comets jostled out of the Oort Comet Cloud (part of the outer solar system) by the gravity of a passing star? We do know that a series of impacts took place 37 million years ago, and is coincident with a substantial period of extinction. Was that a comet shower? We cant say for sure.

Also intriguing is a periodicity of such events, reported by paleontologists Jack Sepkoski and David Raup. They claimed, in 1984, that extinctions occurred in a regular 26 million year pattern. This has been an area of intense debate, and although the argument has abated somewhat, the case was never clearly resolved. Sepkoski continued to publish additional evidence of the periodicity right up to his premature death in 1999.

And what about that silly Nemesis theory? That was an idea published by Marc Davis, Piet Hut, and myself in 1984, designed to account for the periodicity. It said that there was an unseen small companion star orbiting the Sun, presently about one light-year away. I named it Nemesis in honor of the destructive effect it had on the dinosaurs. It hasnt been found yet, so most people assume it doesnt exist. But there have been no measurements that can rule it out. Saul Perlmutter, now famous for his discovery of the accelerating Universe, earned his Ph.D. thesis under my direction for making the initial search.

Some people compare scientists to Diogenes, carrying his lantern in the dark, searching for the truth. Indeed, scientists sometimes feel that way, as they look in dark allies hoping for a discovery lurking behind a trashcan. But I think a more accurate metaphor is that of Sisyphus. According to Greek legend, he was condemned by the gods to roll a boulder up a mountain. He is still at it. Whenever he gets near the top, the rock slips away and rolls back down. Sisyphus will never achieve his goal.

Scientists are like Sisyphus. We make a great discoverywhat killed the dinosaursand over the next quarter-century we experience an explosion of new questions, questions that we never could have imagined asking 25 years earlier.

Albert Camus, the great existentialist philosopher, wrote a great essay called The Myth of Sisyphus. We are all like Sisyphuswe learn, we work, we have children, we die. Thats it. What for? Camus answer: our reward is life itself. The joy is in rolling the rock up the hill. He said, We must conclude that Sisyphus is happy.

Sisyphus had only one boulder roll back down the hill. We scientists are better off. With every discovery comes many new questions to answer. There is a lot more that we dont know now than we didnt know 25 years ago. We are far happier than Sisyphus.