Cartoon of the RCW 120 region where the observer is located on the right and sees a circular shaped H II region in yellow (which is a 3D bubble in reality), and an expanding C⁺ shell (red), surrounded by a molecular ring that is fragmented into clumps (blue). The sketch represents a ‘cut’, there is also molecular material in front of the H II region, this becomes clearer in Fig. 22. CO emission arises from the clump interiors and [C II] from the UV-illuminated clump surfaces and the expanding C⁺ bubble. UV-radiation and X-rays leak into the surrounding molecular cloud and into the enveloping H I cloud (green). This H I layer is our HISA cloud in which carbon is mostly ionized by cosmic rays. The Galactic background (green arrows on the left) is responsible for warm H I.