Lake Baikal, the world's deepest and oldest freshwater lake, is facing one of the most severe ecological crises in recent years. Located in Siberia, it holds about 20% of the world’s non-frozen freshwater and is home to an extraordinary range of unique species, many of which are found nowhere else on Earth. The lake was added to the UNESCO World Heritage list in 1996 due to its immense scientific and ecological value, including its famous endemic species like the Siberian seal, or nerpa, and hundreds of rare shrimp and amphipod species. Its isolation has allowed for the evolution of a distinct ecosystem over millions of years. A major threat to Lake Baikal’s health is the rapid growth of filamentous Spirogyra algae along its shores. This type of algae has been observed accumulating in large quantities—up to 90 kilograms per square meter—in areas previously unaffected. The phenomenon has been recorded in coastal regions near populated areas, a change from its typical occurrence in warmer, shallow bays. Biologist Oleg Timochkine from the Irkutsk Limnological Institute has identified three key areas near cities where the algae has displaced oxygen-dependent mollusks and crustaceans. For example, near the tourist town of Listvianka, underwater forests of sponges that were once abundant have now died, according to Timochkine. The spread of Spirogyra is linked to pollution from wastewater generated by nearby towns and villages. The existing wastewater treatment infrastructure has struggled to keep up with the growing impact of tourism and regional economic development. In Irkutsk, a local environmental group reported that only about 1,600 tons of the 25,000 tons of wastewater produced annually by boats in the region are treated, leaving the rest to flow directly into the lake. This untreated discharge contributes to the algal blooms and degrades water quality. Another challenge comes from the Irkutsk Dam, located approximately 80 kilometers from Lake Baikal on the Angara River, which is the lake’s only natural outlet. This dam, one of three on the Angara River, disrupts the lake’s natural water level fluctuations, which are crucial for maintaining its ecosystem. Current regulations for water level management, set to apply from 2025 to 2027, still allow for significant variations that could harm the lake’s delicate balance. Additionally, Lake Baikal’s water renewal is extremely slow—taking around 383 years on average—which means pollutants like phosphates can remain in the water for centuries. Although a Russian ecological cleanup project ended in 2024, issues like illegal dumping and water pollution persist. A 2025 report to UNESCO confirmed the continued presence of Spirogyra algae and poor water quality in the lake’s tributaries, including the Selenga River, which is its main inflow.