Air containing ~100 ppm H₂ gas is venting about 8 hours a day from multiple locations on the periphery of 550m-diameter barren-of-vegetation depression in the Sao Francisco Basin in Brazil. Atmospheric pressure tides modulate the hydrogen venting in accordion fashion and explain the daily cycle. Modeling indicates that for hydrogen concentrations to change as observed, a substantial, permeable, subsurface reservoir must be tapped by fractures of limited volume. The fractures must have 1/1000^th the volume of the accessed gas reservoir. For the phase of H₂ venting to match the phase of the atmospheric pressure changes, the subsurface reservoir must be terminated by barriers such that the volume decompressed is about 25% of that which could be decompressed. How simple models can probe observations to obtain these conclusions will be the subject of the presentation.