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Revealing the 3D dynamics of HII regions and their associated molecularclouds is important for understanding the longstanding problem as to howstellar feedback affects the density structure and kinematics of theinterstellar medium. We employed observations of the HII region RCW 120 in[CII], observed within the SOFIA legacy program FEEDBACK, and the $^{12}$CO and$^{13}$CO (3$\to$2) lines, obtained with APEX. In addition we used HI data fromthe Southern Galactic Plane Survey. Two radiative transfer models were used tofit the observed data. A line profile analysis with the 1D non-LTE radiativetransfer code SimLine proves that the CO emission cannot stem from aspherically symmetric molecular cloud configuration. With a two-layermulticomponent model, we then quantified the amount of warm background and coldforeground gas. There is a deficit of CO emission along the line-of-sighttoward the center of the HII region which indicates that the HII region isassociated with a flattened molecular cloud. Self-absorption in the CO line mayhide signatures of infalling and expanding molecular gas. The [CII] emissionarises from an expanding [CII] bubble and from the PDRs. A significant part of[CII] emission is absorbed in a cool (~60-100 K), low-density (<500 cm$^{-3}$)atomic foreground layer with a thickness of a few parsec. We propose that theRCW 120 HII region formed in a flattened molecular cloud and is now burstingout of its parental cloud. The compressed surrounding molecular layer formed atorus around the spherically expanding HII bubble. This scenario can possiblybe generalized for other HII bubbles and would explain the observed "flat"structure of molecular clouds associated with HII bubbles. We suggest that the[CII] absorption observed in many star-forming regions is at least partlycaused by low-density, cool, HI-envelopes surrounding the molecular clouds.

Self-absorption in [CII], $^{12}$CO, and HI in RCW120. Building up a geometrical and physical model of the region