A pressure gauge configured to respond to the difference between the ocean pressure and the pressure within a confined volume of compressible oil is found to be especially useful for detecting pressure fluctuations in the frequency range from a few millihertz to a few hertz. In the middle of the range its noise level is lower than any other known gauge. The limitation of the gauge at the lower frequency limit is caused by unpredictable thermal expansion in the confined oil and at the upper limit by thermal agitation noise in the resistance of the strain gauge transducer. The gauge is insensitive to acceleration and tilting. Measurements with this gauge on the deep seafloor show two principal features in the spectrum of pressure fluctuations. At frequencies below 0.03 Hz there is evidence of pressures generated directly by long surface gravity waves. Above 0.11 Hz the pressures associated with microseisms are predominant. Between 0.03 and 0.11 Hz there is a spectral gap where the pressure level drops below 0.1 Pa2 Hz−1.