We report the formation of novel self-assembled ultrathin (ca. 20 nm) films comprising in-plane arrays of nanoscale surface cavities. The original micellar thin films, formed by casting kinetically stable reverse micelles from a solution of a styrene (PS)-acrylic acid (PAA) diblock copolymer in toluene onto silicon nitride substrates, consist of lateral hexagonal arrays of spherical PAA microdomains in PS matrix. Upon aqueous treatment to load metal ions into the PAA domains, cavitation of the individual PAA micelle cores occurs, leading to the formation of cavities in the center of the micelles and exposing the PAA domains to the film's surface. Experimental evidence reveals that cavitation takes place as a result of hydration and swelling of the PAA micelle cores during sample treatment in an alkaline solution. The use of these cavitated, metal-loaded PAA domains as arrays of open nanoreactors for inorganic nanocluster synthesis is demonstrated.