  The 3d Ising model in the low temperature (ferromagnetic) phase describes
dynamics of two-dimensional surfaces -- domain walls between clusters of
parallel spins. The Kramers--Wannier duality maps these surfaces into
worldsheets of confining strings in the Wegner's ${\mathbb Z}_2$ gauge theory.
We study the excitation spectrum of long Ising strings by simulating the
${\mathbb Z}_2$ gauge theory on a lattice. We observe a strong mixing between
string excitations and the lightest glueball state and do not find indications
for light massive resonances on the string worldsheet.
