Time was when the Earth was dominated by dinosaurs who thrived almost everywhere for 150 million years and then abruptly vanished. Why did the biggest and strongest creatures in history die out 65 million years ago?

One persistent theory is starvation: Erupting volcanoes filled the air with so much dust and acid that plants on land and plankton in the seas died.

But what cataclysmic event could have triggered that reaction? The most intriguing explanation -- an asteroid collision -- is explored in absorbing detail tonight in the latest installment of PBS' "NOVA" series (Channels 22 and 26 at 8 p.m.).

The one-hour program picks up geologist Walter Alvarez digging rocks out of a limestone quarry in Gubbio, Italy, where he has found a curious layer of clay separating limestone formations created about 65 million years ago. Walter Alvarez's father, Luis Alvarez -- the University of California's Novel prize-winning physicist -- becomes fascinated at the interruption in the limestone-formation process. "That's the most exciting thing I've ever seen," the older Alvarez tells his son. "Something turned off the limestone for 5,000 years."

Guessing that a major catastrophe must have taken place, Luis Alvarez turns the clay samples over to a team of chemists at Berkeley for neutron activation analysis -- a technique in which soils are bombarded with neutrons, forcing out gamma rays in ways that reveal the soil's composition. The result is a major surprise: The Italian clays are filled with a rare metal called iridium -- an elemental brother of platinum abundant only in meteorites.

"I sat in this chair for six to eight weeks," Luis Alvarez says from his Berkeley office, "and tried to figure out what could have brought in the iridium and killed all these animals." His solution was that an asteroid six miles across struck the Earth with such force that it vaporized itself and an area 50 times its own size. The dust cloud carried into the atmosphere was so great that it turned day into night for the next five years.

"That's the key," Luis Alvarez says. "Without sunlight, plants stopped photosynthesizing. The food chains were disrupted and all the animals died out."

Is that what really happened? "NOVA" asks geologist Gene Shoemaker if asteroid collisions are really that forceful. Looking down into the canyon that is Meteor Crater in Arizona, Shoemaker says: "The meteor that left this hole came down from the sky brighter than the sun, struck the ground with an enormous clap of thunder that produced a cloud of dust that punched up through the atmosphere like a nuclear explosion."

Paleobotanist Leo Hickey, interviewed on location in Wyoming where he is digging up plant samples from 65 million years ago, is skeptical. Not all plants disappeared, says Hickey. And where, he asks, is the crater?

Harvard's Dr. Fred Whipple provides a possible answer. What if the asteroid landed in the ocean, punched through the thin crust on a mid-ocean ridge and tapped the molten lava under the Earth's crust? Whipple says such a collision would have resulted in a colossal volcanic eruption, filling the atmosphere with dust for millions of years and leaving a gigantic land mass in the middle of the ocean.

"I looked all over the ocean ridges and there is only one land mass astride a ridge," Whipple says with a smile. "Iceland. How old is Iceland? Between 50 and 60 million years." Just the right answer to end this scientific detective story.