Tide-induced variations in subsurface air pressure, mainly between -1.0 to 2.0 kPa, are observed in coastal areas in Hong Kong. A two-dimensional model which couples the two-phase air-water flow in unsaturated and saturated zones successfully reproduces the observed air pressure under no rain conditions. The simulation reveals that significant increases in air pressure depend on the tidal rate, not necessarily on the amplitude of the sea level fluctuation. With rainfall, abnormal air pressure 2--9 times greater than without rainfall can be generated when the geological structure, the period and intensity of rain, and the timing of rainfall with respect to the maximum tidal rate combine favorably. This study provides a better understanding of the interactions among the sea tides, barometric pressure variations, groundwater and air-water flow, and rainfall. This natural coastal breathing phenomenon has relevance to the transfer of gases across the soil-seawater-atmosphere interface and to coastal environments around the world.