The world's oldest, deepest and biggest freshwater lake is growing warmer, dirtier and more crowded.

Lyubov Izmestieva is charting these insidious changes. Marina Rikhvanova is fighting them. And the fate of one of the world's rarest ecosystems, a turquoise jewel set in the vast Siberian taiga, hangs in the balance.

For centuries Lake Baikal has inspired wonder and, more recently, impassioned defenders. With more fresh water than the Great Lakes combined, and home to 1,500 species of plants and animals found nowhere else in the world, Baikal has been called Sacred Sea, Pearl of Siberia, Galapagos of Russia.

But these pristine waters, a mile deep in some places, are threatened by polluting factories, a uranium enrichment facility, timber harvesting, and, increasingly, Earth's warming climate. The struggle has turned nasty, with Rikhvanova, an environmental activist, claiming the authorities even dragooned her own son into a violent attack on her group.

Tourists, most of them newly prosperous Russians, are flocking to the lake, filling the beaches, building vacation dachas and changing the lake's ecology. Resorts are opening. There are more fishermen, hunters and boaters.

The lake's significance goes far beyond Russia's borders; its size and fragility, say environmentalists, makes it a sort of test case for such bodies of fresh water around the world.

"Baikal is the greatest lake in the world. It is a limitless reserve and source for water that all of humanity can drink without any sort of purification," says Izmestieva, a third-generation biologist. "This is a priceless gift for everyone, whether you live in Bolshiye Koty or Florida ... or Kansas."

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Shimmering, crystalline waters lap at the hull of the boat named for Izmestieva's scientist grandfather, Mikhail Kozhov, as her colleagues sort plastic jugs and glass bottles and prepare for the day's work.

Lyudmila Ryabenka lowers a plate-sized disc into the rolling waves to measure transparency and quality. Then she winches a cone-shaped net deep into the lake to pull up phytoplankton _ tiny plants that are an essential food source for many fish and shellfish. Later, she and another biologist use a glass cylinder to measure water temperature and collect animal plankton samples.

On the return to the ramshackle village of Bolshiye Koty, Ryabenka says the sampling is sometimes tedious. When the boat pitches or the Siberian winter winds howl, it's even harder. "We say that only romantics do this sort of work."

But every week to 10 days, four seasons a year, for more than 60 years, Izmestiva's family and their colleagues has kept at it.

Izmestiva, 56, the gruff-spoken director of Irkutsk State University's Scientific Research Institute of Biology, is the third generation in her family to do this work. Starting in 1945, her grandfather sailed out onto Baikal's waters _ or trudged out on its ice _ to take samples. When he died, Izmestieva's mother continued the work until her death in 2000. Izmestiva then took over.

Taking the samples became a family ritual, she says. "There's a kind of work that just has to be done whether you like it or not. ... And it's just worked out that we're the ones who have to do it."

The result has been a remarkable trove of data published in the U.S. journal Global Change Biology in an extraordinary paper that concluded Baikal is warming and its food web changing. That echoes other evidence of climate change, including thinning lake ice, arriving later and leaving earlier.

Izmestieva and her colleagues supplement small academic salaries (around $200 a month) consulting for private companies. They store samples in old champagne and vodka bottles. Their work space is the porch of a tired-looking shore-side cabin in Bolshiye Koty.

Now, the university rector wants to rent out the institute's cabins to tourists. That, Izmestieva says, would likely deprive the scientists of a base from which to monitor the lake's changing nature.

"No one will do this if we don't," she says.

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Some 20 to 30 million years ago, scientists believe, a rift in the Earth's crust created Baikal's 400-mile-long, sickle-shaped basin.

Today the lake near the Mongolian border, 2,600 miles east of Moscow, contains one-fifth of the world's fresh water, enough to provide Earth's 7 billion people with six cups of water a day for the next 6,000 years.

It's a sprawling outdoor laboratory of biological diversity comparable to the rich fauna of the Galapagos Islands. Geologists come to study the formation of the Asian continent. Biologists probe such mysteries as how a lake 1,000 miles inland became home to the world's only true species of freshwater seals.

Last month two small, manned submarines reached the bottom of the lake with scientists on board to take soil and water samples. The 5,223-foot dive fell just short of setting a world record.

Baikal inspired the Soviet Union's environmental movement in the 1960s, after Izmestieva's grandfather and other scientists spoke out against Nikita Khrushchev's plans to build a pulp and paper factory on its shores.

Today Marina Rikhvanova, who helped found the nonprofit group Baikal Ecological Wave, is still fighting to close the mill, which has created a dead zone miles wide in the lake and may be contaminating the seals.

A few years back her group led protests against a 2,700-mile oil pipeline, part of which would run along the lake's northern shores. The group's books were audited by authorities, its computers seized and its phones tapped _ retaliation, she says, for fighting the pipeline.

In 2006, then President Vladimir Putin ordered the pipeline rerouted, a rare victory for Russian environmentalists that earned Rikhvanova international accolades. This year she won a prestigious, $150,000 award from the U.S.-based Goldman Foundation.

The 47-year-old former scientist says the victory demonstrates Baikal's potency as a symbol.

The lake "is an indicator of whether modern man can curb his appetite and preserve what nature has created," she says, surrounded by shelves of maps, nature guides and scientific papers. "It's a kind of red line for humanity."

Now she's taking on Kremlin plans to build a uranium enrichment facility 60 miles west of the lake, which would produce nuclear fuel. Officials say the project would bring thousands of jobs to this poor region. Environmentalists say it's a grave mistake that would threaten a natural wonder with radiation.

A year ago Rikhvanova helped organize a tent camp protest not far from the site of the proposed facility. Skinhead nationalists attacked the camp and beat the protesters, one fatally.

Rikhvanova's son, Pavel, was among the intruders, although he denies hurting anyone. She alleges that authorities set up her son in an effort to embarrass her organization. Prosecutors officials refused to comment. Pavel remains in custody.

Despite her personal pain, she says, she is not about to give up. Baikal is too important. "When you see results from your work, you want to continue," she says. "You have to persevere."