  Despite decades of intense researches, the enigmatic pseudo-gap (PG) phase of
superconducting cuprates remains an unsolved mystery. In the last 15 years,
various symmetry breakings in the PG state have been discovered, spanning an
intra-unit cell (IUC) magnetism, preserving the lattice translational (LT)
symmetry but breaking time-reversal symmetry and parity, and an additional
incipient charge density wave breaking the LT symmetry upon cooling. However,
none of these states can (alone) account for the partial gapping of the Fermi
surface. Here we report a hidden LT-breaking magnetism uisng polarized neutron
diffraction. Our measurements reveal magnetic correlations, in two different
underdoped $\rm YBa_{2}Cu_{3}O_{6.6}$ single crystals, that settle at the PG
onset temperature with i) a planar propagation wave vector $(\pi,0) \equiv
(0,\pi)$, yielding a doubling or quadrupling of the magnetic unit cell and ii)
magnetic moments mainly pointing perpendicular to the $CuO_{2}$ layers. The
LT-breaking magnetism is at short range suggesting the formation of clusters of
5-6 unit cells. Together with the previously reported IUC magnetism, it yields
a hidden magnetic texture of the $CuO_{2}$ unit cells hosting loop currents,
forming large supercells which may be crucial for elucidating the PG puzzle.
