Thermokarst lakes, a characteristic landscape element of the Arctic, are rarely found outside arctic situations. Here, a 30-yr photogrammetric monitoring series of a thermokarst lake in the Gruben area, Swiss Alps, is presented. The lake, situated in an environment of dead-ice remains and creeping permafrost, reached a final size of ca. 10,000 m² in area and 50,000 m³ in volume before it had to be drained artificially in 1995. Starting in the mid-1960s it grew with radial rates of ca. 1.5 to 5 m yr⁻¹. Nonlinear coupling of lake diameter and energy turnover led to accelerated area growth. The development of the lake was presumably driven by thermal convection of water. By a dynamic model of lake growth, we show that a change in climate conditions and/or the lake bottom topography could have significantly influenced the observed lake growth. The effective energy turnover used for ice-melt and subsequent lake growth was estimated to be in the order 10⁰ to 10¹ W m⁻².