  The "Nessie" Nebula is a filamentary infrared dark cloud (IRDC) with a large
aspect ratio of over 150:1 (1.5 degrees x 0.01 degrees, or 80 pc x 0.5 pc at a
kinematic distance of 3.1 kpc). Maps of HNC (1-0) emission, a tracer of dense
molecular gas, made with the Australia Telescope National Facility Mopra
telescope, show an excellent morphological match to the mid-IR extinction.
Moreover, because the molecular line emission from the entire nebula has the
same radial velocity to within +/- 3.4 km/s, the nebula is a single, coherent
cloud and not the chance alignment of multiple unrelated clouds along the line
of sight.
  The Nessie Nebula contains a number of compact, dense molecular cores which
have a characteristic projected spacing of ~ 4.5 pc along the filament. The
theory of gravitationally bound gaseous cylinders predicts the existence of
such cores, which, due to the "sausage" or "varicose" fluid instability,
fragment from the cylinder at a characteristic length scale. If turbulent
pressure dominates over thermal pressure in Nessie, then the observed core
spacing matches theoretical predictions. We speculate that the formation of
high-mass stars and massive star clusters arises from the fragmentation of
filamentary IRDCs caused by the "sausage" fluid instability that leads to the
formation of massive, dense molecular cores. The filamentary molecular gas
clouds often found near high-mass star-forming regions (e.g., Orion, NGC 6334,
etc.) may represent a later stage of IRDC evolution.
