  We used a large database extracted from Galaxy-wide surveys of H {\scriptsize
I}, $^{13}$CO 1-0, $8 \micron$ and $870 \micron$ continuum to trace diffuse
atomic gas, low- to medium-density molecular gas, high-density molecular gas,
and star formation activity which we complemented by dedicated $^{12}$CO 2--1,
3--2 observations of the W43 region. From the detailed 3D
(space-space-velocity) analysis of the molecular and atomic cloud tracers
through the region and despite its wide velocity range (\emph{FWHM}$\sim22.3
\kms$ around $\vlsr\sim 95.9 \kms$), we identified W43 as a large (equivalent
diameter $\sim 140$ pc) and coherent complex of molecular clouds which is
surrounded by an atomic gas envelope (equivalent diameter $\sim 290$ pc). We
measured the total mass of this newly-identified molecular complex ($M_{{\tiny
total}}\sim 7.1 \times 10^6 \msun$), the mass contained in dense $870 \micron$
clumps ($<5$ pc dense cloud structures, $M_{{\tiny clumps}}\sim 8.4 \times 10^5
\msun$) and conclude that W43 is particularly massive and concentrated. The
distance we assume for the W43 complex is 6 kpc from the Sun, which may place
it at the meeting point of the Scutum-Centaurus (or Scutum-Crux) Galactic arm
and the Bar, a dynamically complex region where high-velocity streams could
easily collide. The star formation rate of W43 is suggested not to be steady
but it is increasing from $\, \sim 0.01 \msun\, {yr}^{-1}$ (measured from its
$8\,\mu$m luminosity) to $\sim 0.1 \msun\, {yr}^{-1}$ (measured from its
molecular content). From the global properties of W43, we claim it is an
extreme molecular complex in the Milky Way and it could even be forming
starburst clusters in the near future.
