  The discovery of flat bands with non-trivial band topology in magic angle
twisted bi-layer graphene (MATBG) has provided a unique platform to study
strongly correlated phe-nomena including superconductivity, correlated
insulators, Chern insulators and magnetism. A fundamental feature of the MATBG,
so far unexplored, is its high magnetic field Hof-stadter spectrum. Here we
report on a detailed magneto-transport study of a MATBG de-vice in external
magnetic fields of up to B = 31 T, corresponding to one magnetic flux quan-tum
per moir\'e unit cell {\Phi}0. At {\Phi}0, we observe a re-entrant correlated
insulator at a flat band filling factor of {\nu} = +2, and interaction-driven
Fermi surface reconstructions at other fillings, which are identified by new
sets of Landau levels originating from these. These ex-perimental observations
are supplemented by theoretical work that predicts a new set of 8 well-isolated
flat bands at {\Phi}0 , of comparable band width but with different topology
than in zero field. Overall, our magneto-transport data reveals a qualitatively
new Hofstadter spec-trum in MATBG, which arises due to the strong electronic
correlations in the re-entrant flat bands.
