We present a mathematical model describing evolution of false bottoms often met between an under-ice melt pond and the underlying ocean during summer. The model treats a false bottom as the region of mixed phase (mushy layer) whose coordinates depend on time and determine the phase transition area. As the heat and the salt fluxes in the ocean are strongly influenced by turbulence and the ice meltwater accumulating underneath the ice cover is practically fresh, we use modified boundary conditions for heat and mass fluxes at the interfaces of phase transition. Explicit analytical solutions (thickness of false bottom and growth rates of its boundaries, temperature and salinity distributions, solid phase fraction and ocean-to-ice heat flux) of the nonlinear model under consideration are found. Model predictions are in good agreement with existing experimental data and physical concepts of phenomena under study. (C) 2008 Elsevier Ltd. All rights reserved.