  Embedded massive starclusters have recently been identified in several nearby
galaxies by means of the radio-wave thermal bremsstrahlung emission from their
surrounding HII regions. Energy requirements imply that these
optically-obscured starclusters contain 500-1000 O-type stars, making them
similar to the ``super starclusters'' observed in many dwarf starbursts and
mergers. Based on their high free-free optical depth and visual extinctions of
A_V>>10 mag., these massive ``ultra-dense'' HII regions (UDHIIs) are distinct
signatures of the youngest, most compact super starclusters. UDHII regions may
represent the earliest stages of globular cluster formation. We review the
properties of presently-known UDHIIs, and we outline a pictoral evolutionary
taxonomy for massive cluster formation which is analogous to the more familiar
evolutionary sequence for individual stars.
