  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 3-10x10-3 Msol yr-1
are derived.
