Methods and apparatus are provided for passively detecting the presence of near-surface human-scale underground anomalies using earth field measurements. A sensor is used to measure at least one electric or magnetic component of the Earth's electromagnetic field at a frequency of 5 kHz or greater in proximity to the Earth's surface for a given area. The measured intensities are used to identify variations indicative of the presence of a near-surface human-scale underground anomaly. Measuring the intensity of at least one component of the electromagnetic field at a plurality of frequencies of 5 kHz or greater can be used to determine the depth and characteristic of a near-surface human-scale underground anomaly.