Abstract: When rain falls onto a large body of water it produces dominating underwater sound over a broad range of audio frequencies. Laboratory studies using more than 1000 single drops, covering the complete size range of actual rain drops at their terminal speeds, have now shown that the complete underwater spectrum of rainfall sound can be dissected into the impact and microbubble sounds produced by four acoustically distinctive ranges of drop diameters D. These are defined as 'minuscule' drops (D <=0.8 mm), 'small' drops (0.8mm<= D <=1.1mm), 'mid-size' drops (1.1mm<= D <=2.2 mm), and 'large' drops (D >=2.2mm). A minuscule raindrop produces only a very weak, almost undetectable, short duration impact noise. A small drop at terminal speed and at local, near-normal incidence, radiates measurable broadband impact sound followed by the very much stronger sound of a "type I" damped microbubble oscillating at frequencies near 15kHz. A mid-size raindrop radiates only impact sound. Large raindrops, which comprise the major volume of moderate to heavy rainfall, produce an impact sound and a dominating, "type II", "primary" oscillating microbubble of characteristic frequency 2 to 10kHz depending on the drop diameter. Also, large drops often generate weaker sounds from "secondary" bubbles. The average acoustic energy spectra of large raindrops are distinctive functions of their diameters, the salinity of the surface water, and the temperature difference between the drop and the surface water. When the underwater acoustic intensity spectrum during heavy rain is calculated from the single drop acoustic energy spectra and the drop size distribution, it compares quite well with ocean measurements. The gas injection at the air-water interface is calculated from the probability of bubble formation during a heavy rainfall.