  Ultrasound sensors have wide applications across science and technology.
However, improved sensitivity is required for both miniaturisation and
increased spatial resolution. Here, we introduce cavity optomechanical
ultrasound sensing, where dual optical and mechanical resonances enhance the
ultrasound signal. We achieve noise equivalent pressures of 8--300
$\mu$Pa/$\sqrt{\rm Hz}$ at kilohertz to megahertz frequencies in a microscale
silicon-chip-based sensor with $>$120 dB dynamic range. The sensitivity far
exceeds similar sensors that use optical resonance alone and, normalised to
sensing area, surpasses previous air-coupled ultrasound sensors by several
orders of magnitude. The noise floor is, for the first time, dominated by
collisions from molecules in the gas within which the acoustic wave propagates.
This new approach to acoustic sensing could find applications ranging from
biomedical diagnostics, to autonomous navigation, trace gas sensing, and
scientific exploration of the life-induced-vibrations of single cells.
