The acoustic behavior of a water-filled tank 30×12×7 ft deep was investigated at audio frequencies. Such an enclosure differs in theory from the usual reverberant chamber because of the phase reversal in the nearly lossless reflection at the water-air interface. The experimental results differ markedly from the theory. The average absorption coefficient decreases with frequency to a minimum of 0.06 near 5 kc/sec. The number of peaks in the frequency-response characteristic corresponds to only the normal modes tangent to the end planes, as if at least one end wall were completely absorbing. Nevertheless, standing waves do exist in all three dimensions. The standing-wave pattern is extremely complicated and quite unlike the rectangular pattern that mode theory would predict. Considerable sound appears to be transmitted through the concrete walls and floor to the surrounding earth. It is suggested that this transmission may vary greatly from point to point, depending on the inhomogeneous nature of the back fill. It is concluded that a tank of this construction is neither sufficiently reverberant nor sufficiently absorptive to be useful for the calibration of transducers. [The experimental work was performed at the U. S. Navy Electronics Laboratory and supported by the U. S. Office of Naval Research.]