  The confinement of waves in open systems represents a fundamental phenomenon
extensively explored across various branches of wave physics. Recently,
significant attention has been directed towards bound states in the continuum
(BIC), a class of modes that are trapped but do not decay in an otherwise
unbounded continuum. Here, we theoretically investigate and experimentally
demonstrate the existence of quasi-BIC (QBIC) for ultrasonic waves by
leveraging an elastic Fabry-P\'erot metasurface resonator. We unveil several
intriguing properties of the ultrasound QBIC that are robust to parameter
scanning, and we present experimental evidence of a remarkable Q-factor of 350
at around 1 MHz frequency, far exceeding the state-of-the-art using a fully
acoustic underwater system. Our findings contribute novel insights into the
understanding of BIC for acoustic waves, offering a new paradigm for the design
of efficient, ultra-high Q-factor ultrasound devices.
