Context :

Aims : In the framework of multi-wavelength Galactic surveys of star formation which are presently underway , complexes of molecular clouds that stretch over up to hundreds of parsecs are of particular interest .
This is because a large population of stars are forming inside them , thus at the same distance from the Sun and under similar physical conditions .
In the present paper , we focus on the range of the Galactic plane between \approx 29.5 and 31.5 degrees of longitude which is especially rich in terms of molecular clouds and star formation activity .
It is located within what is sometimes called the Molecular Ring and it contains the Galactic mini-starburst region W43 as well as the prominent hot core G29.96-0.02 with its associated compact H II region .

Methods : We used a large database extracted from Galaxy-wide surveys of H I , ^ { 13 } CO 1-0 , 8 ~ { } \mbox { $ \mu$m } and 870 ~ { } \mbox { $ \mu$m } 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 region .

Results : From the detailed 3D ( space-space-velocity ) analysis of the molecular and atomic cloud tracers through the region and despite its wide velocity range ( FWHM \sim 22.3 ~ { } \mbox { km$ $s$ { } ^ { -1 } $ } around \mbox { $V _ { \mbox { \tiny LSR } } $ } \sim 95.9 ~ { } \mbox { km$ $s$ { } ^ { -1 } $ } ) , 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 _ { \mbox { \tiny total } } \sim 7.1 \times 10 ^ { 6 } ~ { } \mbox { $M _ { \odot } $ } ) , the mass contained in dense 870 ~ { } \mbox { $ \mu$m } clumps ( < 5 pc dense cloud structures , M _ { \mbox { \tiny clumps } } \sim 8.4 \times 10 ^ { 5 } ~ { } \mbox { $M _ { \odot } $ } ) 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 ~ { } \mbox { $M _ { \odot } $ } \mbox { yr } ^ { -1 } ( measured from its 8 \mu m luminosity ) to \sim 0.1 ~ { } \mbox { $M _ { \odot } $ } \mbox { 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 .

Conclusions : W43 is the perfect testbed to investigate ( 1 ) the star formation process happening through bursts as well as ( 2 ) the formation of such an extreme complex in the framework of converging flows scenarios .