We have developed a new implosive seismic or acoustic source for seafloor or midwater use. The fact that this device does not use pyrotechnics simplifies logistic and permitting problems. It produces relatively little high-frequency output, so it is wildlife friendly. This device enables us to place the source nearer to the image target compared to surface sources, which thus increases resolution. The simple 20-1 version we have constructed must be reset after each use by bringing it to the sea surface. We present measurements of seafloor shear velocity at a depth of about I km in the San Diego Trough. There the surficial shear velocity is 16 m/s, and the gradient is about 10 s(-1).