Our best mass model of J1721+8842. Left: Magnification map from the z = 2.382 plane. Critical lines in the lens plane are shown in red, while the corresponding inner caustics in the source plane are represented in green. The predicted positions of the images from our model (blue crosses) reproduce the observed image positions (black circles with size proportional to the image’s magnification). The quasar’s position in the source plane is indicated with a star. Right: Visualisation of the optical paths of the lensed images. The two zigzag paths, D and F, are marked in pink and blue, respectively. The deflectors cause two sets of sharp turns, while the smooth curvature seen in all paths is due to the expansion of space.