The fluid dynamics of the well-documented eruptive episodes at Pu’u ’O ’o, Kilauea (Wolfe et al., 1987) are used to investigate quantitatively the size and shape of the shallow conduit system beneath the vent. Cooling calculations are employed to study the long-term survival of conduits and the consequences of multiple dike injection events. We find that the subvent conduit must have a planar geometry at depths greater than a few tens to at most a few hundreds of meters, with a width of less than a few meters, a length of the order of 100 m, and a height of the order of 1 km. This structure is clearly the residue of the preemption dikes. Although such a feature can be widened somewhat by repeated dike emplacement events, there is no evidence to suggest that a larger and much more equant magma reservoir should develop at shallow depth. The extensive degassing often occurring after single eruptions or between repeated eruptions, previously thought to imply the presence of a large equidimensional shallow magma chamber, can be readily explained by the volume of magma in a dikelike subvent planar magma storage zone. On the basis of these observations, which we infer to apply commonly to basaltic eruption sites, we suggest that the terms “chamber” or “reservoir” be used with caution, because they most often connote a relatively large equant magma body, distinct from the feeder dike, lying just below the vent. We find that the subvent region has the same basic geometric characteristics as the parental dike, and we propose that it be referred to as a “planar magma storage zone.”