The interstellar medium (ISM) is the matter (gas 99%, dust 1%) distributed inthe space between the stars. Hydrogen (H) is the most abundant element of theISM, and its physical/chemical state defines the ISM phases: the ionised phase,in which H is atomic ionised, the neutral phase, in which H is neutral atomic,and the molecular phase, in which H is neutral molecular (H2). Star formationis the set of physical processes that forms stars from the material of the ISM. Inparticular, molecular clouds are the places where stars are born. The largestmolecular clouds are called Giant Molecular Clouds (GMCs), with typicaldensities of 102–103cm?3, diameters greater than 25 pc, average temperaturesof 10 K and masses of about 105M?(e.g.Murray2011). Turbulence, gravityand magnetic field give rise to dense cores of different mass and size throughthe fragmentation process inside a large GMC. Cores have typical sizes of0.02–0.2 pc, masses of?0.5–5 M?and densities>104cm?3(e.g.Williamset al.2000;Bergin and Tafalla2007). Testing the theories presented above with observations is challenging forseveral reasons: being at distances?1 kpc massive star-forming regions havesmall angular sizes, they are few with respect to low-mass stars, and they areusually embedded in crowded environments (e.g.Zinnecker and Yorke2007;Tan et al.2019). Nevertheless, investigations of the natal environment of mas-sive stars provide clues for a better understanding of its physical and chemicalproperties and of the star formation process itself.