When a liquid is superheated above its boiling point to temperatures near or at the homogeneous nucleation limit, the energy released could create a so-called explosive vaporization, if a significant fraction of this energy is manifested in the form of vapor expansion. In this study, a thin-film microheater (100 mu m x 110 mu m) was placed on the underside of a water layer. The surface temperature of the heater was rapidly (6 mu s) raised electrically, well above the boiling point of water. As a result, rapid vaporization took place. Due to its rapid growth, the vapor volume performs mechanical work on its surrounding and emits acoustic pressure waves. By measuring the acoustic emission from an expanding volume, the dynamic growth of the vapor microlayer is reconstructed where a linear expansion velocity up to 17 m/s was reached. Using the Rayleigh-Plesset equation? an absolute pressure inside the vapor volume of 7 bar was calculated from the data of the acoustic pressure measurement. The amount of extractable mechanical energy produced from the explosive expansion of a vapor microlayer on a thin-film microheater surface? its rate of production, and the energy conversion efficiency was also quantified in this work.