  In this paper we consider the six-vertex model at ice point on an arbitrary
three-bundle domain, which is a generalization of the domain-wall ice model on
the square (or, equivalently, of a uniformly random alternating sign matrix).
We show that this model exhibits the arctic boundary phenomenon, whose boundary
is given by a union of explicit algebraic curves. This was originally predicted
by Colomo-Sportiello in 2016 as one of the initial applications of a general
heuristic that they introduced for locating arctic boundaries, called the
(geometric) tangent method. Our proof uses a probabilistic analysis of
non-crossing directed path ensembles to provide a mathematical justification of
their tangent method heuristic in this case, which might be of independent
interest.
