Linda Zall played a starring role in science that led to decades of major advances. But she never described her breakthroughs on television, or had books written about her, or received high honors. One database of scientific publications lists her contributions as consisting of just three papers.

The reason is that Zall's decades of service to science were done in the secretive warrens of the CIA.

Now, at 70, she's telling her story — at least the parts she is allowed to talk about — putting the nation's spy satellites onto a radical new job: environmental sleuthing.

Zall's program, established in 1992, was a kind of wayback machine that looked to as long ago as 1960. In so doing, it provided a new baseline for assessing the pace and scope of planetary change. The accumulated riches included up to six decades of prime data on planetary shifts in snowfall and blizzards, sea ice and glaciers.

"None of this would have happened without her," said Jeffrey K. Harris, who worked with Zall as director of the National Reconnaissance Office, which runs the nation's fleet of orbital spies.

The top-secret images that Zall succeeded in repurposing for environmental inquiries came from satellites that were some of Washington's crown jewels. The spy satellites would zero in on such targets as deadly weapons and render images that in some cases were said to be good enough to show a car's license plate. The first reconnaissance satellite, known as Corona, was launched in 1960. Federal experts have put the overall cost of its hundreds of successors at more than $50 billion.

Many spy satellites orbit on north-south paths that pass close to the poles. Spies had little use for the sweeping images. But they dazzled environmentalists because it showed the extent to which Arctic and Antarctic ice was retreating.

"It gave us the first real measurements of the ice budget — how much loss you have from season to season," said D. James Baker, who directed the National Oceanic and Atmospheric Administration from 1993 to 2001 and served on Zall's CIA advisory panel.

Zall's environmentalism for the CIA began in 1990 when then-Sen. Al Gore asked the agency to examine whether the nation's spy fleet might address environmental riddles. Zall quickly saw how the nation's archive of surveillance observations could also serve to strengthen assessments of Earth's changing environment.

Zall, who graduated from Cornell University in 1976 with a doctorate in civil and environmental engineering, started her career working for the Earth Satellite Corp., which used computers to enhance civilian surveillance Landsat images taken for farmers, geographers and other specialists.

After 1984, Zall vanished into the CIA, using her skills to improve the analysis of reconnaissance images and to plan new generations of spy satellites. Then, in late 1991, the Soviet Union disintegrated, diminishing a top rationale for maintaining a fleet of costly spy satellites.

Zall wrote a highly classified report describing what the secret reconnaissance could do for Earth science. By October 1992, the CIA was so confident in the ability of spy satellites to solve environmental mysteries that it established a task force, putting Zall in charge.

As part of the post-Cold War thaw, the Clinton administration wanted to engage Russia with new projects. The Soviets, it turned out, had amassed a treasure of Arctic ice data. The negotiations to share the trove involved top officials from both sides, starting with Zall. "I went to Moscow probably 10 times and St. Petersburg twice," she said.

In 1995, the group's work was the driving force when President Bill Clinton ordered the declassification of more than 800,000 spy-satellite images. Taken from 1960 through 1972, the images showed not only airfields but also swaths of land marked by deforestation and environmental ills. A 1962 image revealed the Aral Sea before an ecological catastrophe left it bone dry.

The task force also fostered a parallel movement for the Navy to release once-secret information that illuminated inner space — the ocean's sunless depths. In late 1995, a new map of the seabed was unveiled that bared riots of deep fissures, ridges and volcanoes. "This was the first, uniform map of the global seafloor," said John A. Orcutt of the Scripps Institution of Oceanography in San Diego.