For this challenge we got an .stl file, wich holds a 3D object, in this case a big christmas ball ornament. As I didn't want to install blender and the countless online STL viewers couldn't edit the ornament to peek inside, I used 3D Builder, a tool that comes with Windows 10 and which worked reasonably well.
I used the "split" function to cut off the top and bottom parts of the ornament to reveal the 2D barcode hidden inside:
I then took a screenshot looking straight down on the structure, opened it in Gimp and applied thresholding to convert it into a relatively clean looking 2D barcode:
Barcode enthusiasts will quickly recognize this as an Aztec code. I used the ZXing online decoder to read the flag:
