  Second-order superlattices form when moir\'e superlattices of similar
periodicities interfere with each other, leading to even larger superlattice
periodicities. These crystalline structures have been engineered utilizing
two-dimensional (2D) materials such as graphene and hexagonal boron nitride
(hBN) under specific alignment conditions. Such specific alignment has shown to
play a crucial role in facilitating correlation-driven topological phases
featuring the quantized anomalous Hall effect. While signatures of second-order
superlattices have been identified in magnetotransport experiments, any
real-space visualization is lacking to date. In this work, we present
\NT{electronic transport measurements and cryogenic nanoscale photovoltage (PV)
measurements} that reveal a second-order superlattice in magic-angle twisted
bilayer graphene closely aligned to hBN. This is evidenced by long-range
periodic photovoltage modulations across the entire sample backed by the
corresponding electronic transport features. Supported by theoretical
calculations, our experimental data show that even minuscule strain and
twist-angle variations on the order of 0.01$^\circ$ can lead to a drastic
change of the second-order superlattice structure between local
one-dimensional, square or triangular types. Our real-space observations
therefore serve as a strong `magnifying glass' for strain and twist angle and
can shed new light on the mechanisms responsible for the breaking of spatial
symmetries in twisted bilayer graphene, and pave an avenue to engineer
long-range superlattice structures in 2D materials using strain fields.
