  We discuss the sensitivity of a 100 TeV $pp$ collider to heavy resonances
produced in longitudinal vector boson scattering and decaying to a pair of
Higgs bosons. A Monte Carlo study has been performed using the $H \to \tau
\tau$ decay channel for both Higgs bosons, comparing the kinematics of such a
signal to the irreducible Standard Model backgrounds. The results are presented
in the context of a phenomenological model of a resonance ($\eta$) coupling to
goldstone modes, $V_L V_L \to \eta \to HH$, as can arise in composite Higgs
models. With a fractional width of 70% (20%), the $5 \sigma$ discovery reach is
4.2 (2.9) TeV in resonance mass for 10 ab$^{-1}$ of integrated luminosity. We
also discuss the dependence of the mass reach on the collider energy and
integrated luminosity.
