Switzerland just activated Nant de Drance Phase 3 — a 900-megawatt underground pumped hydro expansion drilled 600 meters into Valais Alpine granite, the world's deepest underground hydroelectric cavern and the largest new energy storage facility opened in Europe this decade.

 

Switzerland just activated Nant de Drance Phase 3 — a 900-megawatt underground pumped hydro expansion drilled 600 meters into Valais Alpine granite, the world's deepest underground hydroelectric cavern and the largest new energy storage facility opened in Europe this decade.

The expansion excavated a lower reservoir chamber 600 meters below the existing Lac des Toules upper reservoir, increasing hydraulic head from 390 to 600 meters. Greater depth stores more energy per cubic meter of water — Phase 3 stores 20 million kilowatt-hours from the same volume that Phase 2 stored 13 million, a 54 percent improvement without additional surface footprint.

Switzerland faces extreme seasonal imbalance — excess summer hydroelectric generation and winter import dependency. Nant de Drance Phase 3 provides storage covering Switzerland's entire national electricity demand for 6 continuous hours, resolving the seasonal gap without any new dam.

Source: Nant de Drance SA Switzerland, Swiss Federal Office of Energy, Swissgrid National Grid Operator, 2025

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