Cloud contraction and infall are the fundamental processes of star formation .
While “ blue-skewed ” line profiles observed in high-mass star forming regions are commonly taken as evidence of infall by an ever increasing number of studies , their interpretation offers many pitfalls .
Detecting infall via redshifted absorption in front of continuum sources is a much more direct and reliable method but so far mostly restricted toward absorption in the centimeter toward strong HII regions .
Here we present a novel approach by probing absorption of rotational ammonia transitions in front of the strong dust emission of massive star-forming regions .
A carefully selected sample of three regions with different stages of evolution is selected to study infall through the evolution of massive star-forming clumps .
Redshifted absorption is detected toward all three sources and infall rates between 3 - 10 \times 10 ^ { -3 } M _ { \odot } yr ^ { -1 } are derived .