Observations of the Most Massive Deeply Embedded Star Clusters in the
  Milky Way
  We summarize our comprehensive gas surveys of some of the most luminous,
deeply embedded (optically obscured) star formation regions in the Milky Way,
which are the local cases of massive star clusters and/or associations in the
making. Our approach emphasizes multi-scale, multi-resolution imaging in dust
and free-free continuum, as well as in molecular- and hydrogen recombination
lines, to trace the multiple gas components from 0.1 pc (core scale) all the
way up to the scales of the entire giant molecular cloud (GMC), or $\sim 100$
pc. We highlight our results in W49A, the most luminous Galactic star formation
region ($L\sim 10^7~L_\odot$), which appears to be forming a young massive
cluster (or a binary star cluster) with $M_\star > 5\times10^4~M_\odot$ that
may remain bound after gas dispersal. The surveyed sources share elements in
common, in particular, the 10-100 pc scale GMCs are filamentary but have one or
two central condensations (clumps) far denser than the surrounding filaments
and that host the (forming) massive stars.
