  Young massive clusters (YMCs) with stellar masses of 10^4 - 10^5 Msun and
core stellar densities of 10^4 - 10^5 stars per cubic pc are thought to be the
`missing link' between open clusters and extreme extragalactic super star
clusters and globular clusters. As such, studying the initial conditions of
YMCs offers an opportunity to test cluster formation models across the full
cluster mass range. G0.253+0.016 is an excellent candidate YMC progenitor. We
make use of existing multi-wavelength data including recently available far-IR
continuum (Herschel/Hi-GAL) and mm spectral line (HOPS and MALT90) data and
present new, deep, multiple-filter, near-IR (VLT/NACO) observations to study
G0.253+0.016. These data show G0.253+0.016 is a high mass (1.3x10^5 Msun), low
temperature (T_dust~20K), high volume and column density (n ~ 8x10^4 cm^-3;
N_{H_2} ~ 4x10^23 cm^-2) molecular clump which is close to virial equilibrium
(M_dust ~ M_virial) so is likely to be gravitationally-bound. It is almost
devoid of star formation and, thus, has exactly the properties expected for the
initial conditions of a clump that may form an Arches-like massive cluster. We
compare the properties of G0.253+0.016 to typical Galactic cluster-forming
molecular clumps and find it is extreme, and possibly unique in the Galaxy.
This uniqueness makes detailed studies of G0.253+0.016 extremely important for
testing massive cluster formation models.
