  Light-matter interaction in the ultrastrong coupling regime can be used to
generate exotic ground states with two-mode squeezing and may be of use for
quantum enhanced sensing. Current demonstrations of ultrastrong coupling have
been performed in fundamentally nonlinear systems. We report a cavity
optomechanical system that operates in the linear coupling regime, reaching a
maximum coupling of $g_x/\Omega_x=0.55\pm 0.02$. Such a system is inherently
unstable, which may in the future enable strong mechanical squeezing.
