Let be a domain in R 3 with ∂ = ∂(R 3 \ ), where ∂ is unbounded and connected, and let u be the solution of the Cauchy problem for the heat equation ∂ t u = u over R 3 , where the initial data is the characteristic function of the set c = R 3 \ . We show that, if there exists a stationary isothermic surface of u with ∩ ∂ = ∅, then both ∂ and must be either parallel planes or co-axial circular cylinders . This theorem completes the classification of stationary isothermic surfaces in the case that ∩ ∂ = ∅ and ∂ is unbounded. To prove this result, we establish a similar theorem for uniformly dense domains in R 3 , a notion that was introduced by methods from the theory of surfaces with constant mean curvature, combined with a careful analysis of certain asymptotic expansions and a surprising connection with the theory of transnormal functions.