Starting at an initial density of air, suppose you descend a distance D such that the air density doubles. Now your air is twice as dense, which doubles the pre

Starting at an initial density of air, suppose you descend a distance D such that the air density doubles. Now your air is twice as dense, which doubles the pressure underneath it, meaning if you descend a further D the density will double again. Continue ad infinitum (or at least until the ideal gas law stops being a good approximation).