During the dry season of the year 2002, convective cooling triggered the formation of a double-diffusive regime of the convective type below the chemocline at 52m depth in Lake Nyos. Since then, the vertical extent of the double-diffusive staircase has continuously increased, and the front reached a depth of about 92m in March 2004. The input of turbulent energy from wind or surface convection below the chemocline is very weak and not sufficient to disturb the development of the double-diffusive staircase. The lake represents thus a perfect natural laboratory to study the expansion and the horizontal variability of a double-diffusive staircase at a much larger scale than in laboratory experiments. Between March 2002 and March 2004, the staircase expanded vertically with a seemingly rather constant average speed of about 1.3m per month. The formation of the double-diffusive staircase temporarily leads to a local divergence of the vertical heat flux, which also propagates downwards. The merging of layers in the upper part of the double-diffusive zone was about as frequent as the formation of new layers, keeping the number of observed layers fairly constant.