Sketch of the OMC-1/2/3 region as it might have appeared before the formation of the Trapezium cluster (top), and how it looks at present time (bottom). The foreground cloud we detected is labelled “fossil cloud”, and the colour gradient across it represents a density gradient roughly in the north-south direction (see text). The sizes of each component and their velocities relative to the systemic velocity of OMC-1 and the foreground cloud (i.e. V_LSR = 9 km s^-1) are marked. The observer, represented by a small picture of the Herschel satellite, is situated to the right of each panel.