When Mark Twain visited Lake Tahoe in 1861, he was so entranced by the sky blue transparency of its depths that he likened his boat rides to "balloon voyages." Nestled between the Sierra Nevada and the Carson Range on the California-Nevada border, Tahoe, among the world's deepest lakes at 1,645 feet, still awes visitors with its clarity. Yet old-timers and scientists can see a difference: Tahoe is clouding up. Monitoring by researchers at the University of California, Davis, shows that clarity has diminished by a third. People could see to an average depth of 102 feet in 1968 but only to 70 feet today. Light-scattering sediments carried by runoff from condos, marinas, and other growth on the shore have steadily dimmed visibility. Warming of the lake due to climate change could also dull the appearance because of a shift in nutrient mixing. Armed with data, partisans are seeing results from a Keep Tahoe Blue campaign. Additional runoff controls are planned for 2009. Tahoe can recover quickly, scientists believe, and attain the diamond-like clarity of old.