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K. Johnson (NRAO/U.Wisconsin)
We have recently discovered a number of extragalactic star clusters which are still deeply enshrouded in their natal material. These clusters appear to have properties which are analogous to those of ultracompact HII regions in the Galaxy, albeit vastly scaled up. Furthermore, a continuum of young embedded clusters have been found ranging between small OB associations to super star clusters, with the upper mass limit correlated to the overall star formation rate of the host galaxy. Invisible at optical and near-IR wavelengths, these clusters are generally identified in the radio regime as having an optically thick free-free spectral energy distribution at wavelengths < 6~cm (\alpha > 0 for S\nu \propto \nu \alpha). A subset of sources observed with Gemini are luminous in the mid-IR, confirming the warm dust cocoons surrounding the ultra-young clusters. The inferred properties of these objects are truly remarkable: they have estimated sizes of a few parsecs, masses of up to ~106~M\odot, electron densities up to ~105~cm-3, and ages possibly as young as a few \times 105~years. These properties imply that we may be witnessing the earliest stages of globular cluster evolution yet observed.