  The Hofstadter butterfly is one of the first and most fascinating examples of
the fractal and self-similar quantum nature of free electrons in a lattice
pierced by a perpendicular magnetic field. However, the direct experimental
verification of this effect on single-layer materials is still missing as very
strong and inaccessible magnetic fields are necessary. For this reason, its
indirect experimental verification has only been realized in artificial
periodic 2D systems, like moir\'e lattices. The only recently synthesized 2D
covalent-organic frameworks might circumvent this limitation: Due to their
large pore structures, magnetic fields needed to detect most features of the
Hofstadter butterfly are indeed accessible with today's technology. This work
opens the door to making this exotic and theoretical issue from the 70s
measurable and might solve the quest for the experimental verification of the
Hofstadter butterfly in single-layer materials. Moreover, the intrinsic
hierarchy of different pore sizes in a 2D covalent-organic framework adds
additional complexity and beauty to the original butterflies and leads to a
directly accessible playground for new physical observations.
