Using a regional upper-mantle model with an unprecedented spatial resolution of ∼100 m, we have investigated at multiple resolutions the character of incipient magma chambers forming under oceanic arcs. The magma chambers are formed from wave-like structures propagating upward along descending slabs and consist of compositionally buoyant, hydrated, partially molten subducted crustal and mantle material. These wave structures are 300–500 °C colder than the mantle wedge and may have an upward velocity of >1 m/yr. Inverted temperature structures and transitory bimodal magmatism are plausible consequences of finger-like penetration of relatively cold, hydrated material of the incipient magma chambers into the hot mantle wedge. Apart from forming and periodic feeding of the magma chambers, “cold” waves may also transport upward thousands of cubic kilometers of subducted material and may cause the rapid exhumation of ultrahigh-pressure rocks along slabs.