Zoom-in from Fig. 4, showing the location of the spectroscopically observed galaxies. The contours of the 250 μm Herschel/SPIRE map are over-plotted in magenta. These galaxies have been colour-coded according to their spectroscopic redshifts (also included in the figure), from dark blue (closest galaxy, i.e. A4c, z = 1.615) to red (furthest, A5, z = 2.121). The members of the two high-z structures are then shown in cyan (z ≃ 1.7) and yellow (z ≃ 2.0). The galaxies for which we could not determine their spectroscopic redshifts (B1, B4, C2, D2, and D3) are marked in white. For reference, a circle of 1′ radius, centred at RA = 00:00:43.31 and Dec = − 01:25:50.5, is shown in white. In addition, the scales corresponding to a comoving size of 1 Mpc at z ≃ 1.7 and z ≃ 2.0 are also shown in the bottom left corner. North is up and east is left.