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It is a long-lasting question to which extent expanding HII regions, that appear circular in IR images, are indeed 3D bubbles. We investigated the 3D structure of the archetypical HII region RCW 120, using [CII] 158 μm observations from the SOFIA FEEDBACK legacy project, 12CO and 13CO (3-2) lines from APEX, and HI and continuum data from the CGPS. The relatively small intensity difference of the primary isotope (12C) to the secondary (13C) compared to the local isotopic ratio and the strong absorption dips in spectral structure of the primary isotopes suggest strong self-absorption effects in the [12CII] and 12CO (3-2) lines. To disentangle the self-absorption features from the “true” spectral shape, we solve the radiative transfer equations for two layers. Hereby we obtained that a substantial amount of the C+ mass is hidden in a cold absorbing foreground layer. From HI self-absorption studies (HISA), we find that this surrounding layer is atomic with T ~ 15-30K, has a low density ~100-500 cm-3 and an extend of about ~ 5-10 pc. To solve the two-layer model for the entire CO data cube we apply a Gaussian mixture model to cluster the data cube by spectra with similar spectral shapes. This allows us to initialize the model for an entire spectral cube with a few initial parameters. The model results suggest that RCW 120 developed out of a flat molecular cloud, enveloped by HI, and most of the CO is located inside the torus compressed by the stellar wind from the central star. The sudden morphological change in the cold foreground layer towards red-shifted velocities might indicate that the torus is still in formation and accreting mass.