Because of the program’s popularity, he no longer advertises it. Most of the volunteers have devotedly returned for every project and become a highly skilled team.

During their first week, in 2001, volunteers searched along the lower edges of the canyon walls that frame the valley for the bluish-white fossils that stand out among the brown and gray rocks and bright yellow grasses — “developing their bone goggles,” Dr. Schumacher calls it.

By the second-to-last day, they’d found nothing. So one volunteer took from his knapsack some petrified wood he had brought back to show his dejected partners.

“That’s not wood,” Dr. Schumacher recalled telling the volunteer, who led the team back to where he had found the fossil.

There, over the next several seasons, the crew spent a week or two every year uncovering a Camarasaurus that they named Woody. In all, they pulled out about 15 percent of the skeleton, now on display in the Sternberg Museum of Natural History at Fort Hays State University in Kansas.

Most of the volunteers are amateurs interested in paleontology, like a retired meat cutter, a retired secretary of an oil and gas company and a retired aerospace engineer. Some did their own bone hunting before they found the program. Others were looking for opportunities for travel-oriented post-retirement volunteer work; many have worked on similar archaeological projects around the world.