  The ubiquity of filaments in star forming regions on a range of scales is
clear, yet their role in the star formation process remains in question. We
suggest that there are distinct classes of filaments which are responsible for
their observed diversity in star-forming regions. An example of a massive
molecular filament in the Galactic mid-plane formed at the intersection of
UV-driven bubbles which displays a coherent velocity structure (< 4 km/s) over
80 pc is presented. We classify such sources as Massive Molecular Filaments
(MMFs; M > 10^4 Msun, length > 10 pc, velocity gradient < 5 km/s) and suggest
that MMFs are just one of the many different classes of filaments discussed in
the literature today. Many MMFs are aligned with the Galactic Plane and may be
akin to the dark dust lanes seen in Grand Design Spirals.
