Infrared dark clouds on the far side of the Galaxy
  Context: Infrared dark clouds are the coldest and densest portions of giant
molecular clouds. The most massive ones represent some of the most likely
birthplaces for the next generation of massive stars in the Milky Way. Because
a strong mid-IR background is needed to make them appear in absorption, they
are usually assumed to be nearby.
  Aims: We use THz absorption spectroscopy to solve the distance ambiguity
associated with kinematic distances for the IR-dark clouds in the TOP100
ATLASGAL sample, a flux-limited selection of massive clumps in different
evolutionary phases of star formation.
  Methods: The para-H2O ground state transition at 1113.343 GHz, observed with
Herschel/HIFI, was used to investigate the occurrence of foreground absorption
along the line of sight directly towards infrared-dark clouds. Additional
consistency checks were performed using MALT90 and HiGAL archival data and
targeted Mopra and APEX spectroscopic observations.
  Results: We report the first discovery of five IRDCs in the TOP100 lying
conclusively at the far kinematic distance, showing that the mere presence of
low-contrast mid-IR absorption is not sufficient to unequivocally resolve the
near/far ambiguity in favour of the former. All IRDCs are massive and actively
forming high-mass stars; four of them also show infall signatures.
  Conclusions: We give a first estimate of the fraction of dark sources at the
far distance (~11% in the TOP100) and describe their appearance and properties.
The assumption that all dark clouds lie at the near distance may lead, in some
cases, to underestimating masses, sizes, and luminosities, possibly causing
clouds to be missed that will form very massive stars and clusters.
