This excerpt from Digging Snowmastodon: Discovering an Ice Age World in the Colorado Rockies by Kirk Johnson and Ian Miller describes research featured in NOVA's Ice Age Death Trap, premiering Wednesday, February 1 at 9 p.m. ET on most PBS stations.

The digging team celebrates project completion. Courtesy Kirk Johnson

On September 20, the day after my birthday, a Gould Construction Inc. crew began to push dirt at the Ziegler Reservoir construction site near Snowmass Village in the Colorado Rockies. They had two months to dig the footing that would change a small lake into deep reservoir. They were using D6 Cats and big track hoes, loading huge dump trucks to haul away the dirt. Work progressed smoothly, with the trucks making dozens of trips each day and creating an ever-deepening hole.

Kent Olson, Gould's on-site foreman, found a brown bone while walking across the site and had that odd feeling that contractors get when they find bones. He talked about the bone with his boss, Mark Gould, and they showed it to Bob Mutaw, an archaeologist who worked for URS, the engineering firm that was overseeing the site. Mutaw looked over the bone and pronounced it bovine, probably an old milk cow. Kent wasn't so sure. The work continued, but the workers started making nervous jokes about old bones. Kent even played a practical joke by wrapping a big log in black plastic and sticking it on the tailgate of his project manager's truck. Then he casually mentioned that he had found a dinosaur bone. Not funny.

Gould's number-one dozer operator was Jesse Steele from Palisade, Colorado. Jesse is a polite, compact cowboy who wears a black hat and tips it when he greets a lady. He is also a third-generation dozer operator. As a toddler, he dozed in his grandfather's lap, in a dozer. He first drove a dozer at the age of five. When it comes to moving dirt, Jesse is a smooth operator.

At about four in the afternoon of October 14, Jesse was operating his D6, pushing through a thick brown layer of organic soil known as peat, when a pair of giant ribs flicked over the top of the blade. Jesse stopped the machine and hopped out to take a look. The ground in front of his blade was littered with big brown bones. Instead of getting excited, Jesse got scared.

Kent came over and together they began to gather the bones. They found a partial jawbone with an 8-inch-long tooth. They found a tusk. They found big vertebrae. It was clear that this was a big skeleton. Joe Enzer came over to the find, took one look, turned to Kent and said, "This is not a cow, and there is no way we can ever call it a cow." Kent took the bones home that night and got on the Internet. It didn't take him long to realize that Jesse had run over the skeleton of a mammoth.

This book is the story of what happened over the next nine months as Jesse's mammoth turned into the most significant high-elevation ice age fossil site in the world and the biggest fossil dig in Colorado history.

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Thursday, November 4 - Wednesday, November 10, 2010

On November 6, several more geologists and paleontologists arrived on site. With their additional brain power we debated how and when this glacial lake had formed and how long it had lasted. We were still waiting for the results from the radiocarbon samples that we had sent to Florida, and we were all still operating under the assumption that the quality of preservation suggested that the site couldn't be much older than 13,000 years.

We now had a pretty big crew of scientists and volunteers on site and were focusing our efforts on excavating the mammoth under the tent. We had smaller crews digging down in the hole where the mastodon and sloth bones had been found. We were in general agreement that the silt layer between the moraine and the peat was barren of fossils. Mark Gould, who had supervised the excavation of nearly 80,000 yards of sediment over the last month, was convinced that his guys had seen no bones in the silt.

Early in the afternoon, this conviction was changed by a dramatic event. Jesse was slowly pushing his dozer through the silt layer at the bottom of the hole with Dane and Ian running blade. Just below the tent, the dozer unearthed a 3-foot-long bone that initially looked like another tusk. Upon close inspection, we realized that the bone was the core of an absolutely immense bison horn. It had been broken into three pieces and there were fresh breaks, indicating that the horn had been sheared from a skull. The pieces exposed the center of the horn, which was formed of a coarse, butterscotch-colored honeycomb latticework. It looked good enough to eat.

We stopped the dozer and spread out with shovels, trying to find the skull. Eight of us looked for the better part of two hours with absolutely no luck. Finally we gave up, grudgingly deciding that the horn must have been a solitary fragment. With that decision, we asked Jesse to fire up the dozer and take another next pass. Amazingly, this time the dozer pushed up a second immense horn. And this time we were able find the spot in the silt where the horn had come from. After an hour of shoveling, we uncovered an incredibly large skull. Both horns fit back to the skull, and we came face-to-face with a huge bison. Productivity dropped way down as the entire crew gathered around to watch the beast emerge from the silt.

We carefully wrapped the two horns and applied burlap and plaster to the giant skull. It was hard to tell that day, but when measured, the skull was an amazing 6 foot 4 inches from broken horn tip to broken horn tip.

The bison discovery prompted volunteers Bill and Judy Peterson to hand me a $100 bill, the first financial contribution to the project. It had also profoundly affected Cathy Dea, who had helped to encase the skull in plaster, and in the process had coated herself in plaster and mud. She looked like a muddy urchin, but the look on her face was one of rapturous delight. A good fossil can do that to you.

For Russ Graham, an expert in ice age bison, this big beast rang some bells. Based on his knowledge, the big-horned bison went extinct more than 40,000 years ago. Russ suggested that our idea that the site was only 13,000 years old was probably wrong. While waiting for the radiocarbon dates to come back in a few days, people started to make wagers about the age of the site.

This was only the first of several mysteries that would appear as the huge excavation stretched over 70 days. NOVA's Ice Age Death Trap chronicles what happened next.

Ice Age Death Trap premieres Wednesday, February 1 at 9 p.m. ET on most PBS stations. Please check your local listings to confirm when it will air near you.