Filaments play a central role in the molecular clouds evolution, but their internal dynamical properties remain poorly characterized. To further explore the physical state of these structures, we have investigated the kinematic properties of the Musca cloud. We have sampled the main axis of this filamentary cloud in $^{13}$CO and C$^{18}$O (2--1) lines using APEX observations. The different line profiles in Musca shows that this cloud presents a continuous and quiescent velocity field along its $sim$6.5 pc of length. With an internal gas kinematics dominated by thermal motions (i.e., $sigma_{NT}/c_slesssim1$) and large-scale velocity gradients, these results reveal Musca as the longest velocity-coherent, sonic-like object identified so far in the ISM. The transonic properties of Musca present a clear departure from the predicted supersonic velocity dispersions expected in the Larsons velocity dispersion-size relationship, and constitute the first observational evidence of a filament fully decoupled from the turbulent regime over multi-parsec scales.
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