  Chipscale micro- and nano-optomechanical systems, hinging on the intangible
radiation-pressure force, have shown their unique strength in sensing, signal
transduction, and exploration of quantum physics with mechanical resonators.
Optomechanical crystals, as one of the leading device platforms, enable
simultaneous molding of the band structure of optical photons and microwave
phonons with strong optomechanical coupling. Here, we demonstrate a new breed
of optomechanical crystals in two-dimensional slab-on-substrate structures
empowered by mechanical bound states in the continuum (BICs) at 8 GHz. We show
symmetry-induced BIC emergence with optomechanical couplings up to
$g/2\pi\approx 2.5$ MHz per unit cell, on par with low-dimensional
optomechanical crystals. Our work paves the way towards exploration of
photon-phonon interaction beyond suspended microcavities, which might lead to
new applications of optomechanics from phonon sensing to quantum transduction.
