  The study of moir\'e engineering started with the advent of van der Waals
heterostructures in which stacking two-dimensional layers with different
lattice constants leads to a moir\'e pattern controlling their electronic
properties. The field entered a new era when it was found that adjusting the
twist between two graphene layers led to strongly-correlated-electron physics
and topological effects associated with atomic relaxation. Twist is now used
routinely to adjust the properties of two-dimensional materials. Here, we
investigate a new type of moir\'e superlattice in bilayer graphene when one
layer is biaxially strained with respect to the other - so-called biaxial
heterostrain. Scanning tunneling microscopy measurements uncover spiraling
electronic states associated with a novel symmetry-breaking atomic
reconstruction at small biaxial heterostrain. Atomistic calculations using
experimental parameters as inputs reveal that a giant atomic swirl forms around
regions of aligned stacking to reduce the mechanical energy of the bilayer.
Tight-binding calculations performed on the relaxed structure show that the
observed electronic states decorate spiraling domain wall solitons as required
by topology. This study establishes biaxial heterostrain as an important
parameter to be harnessed for the next step of moir\'e engineering in van der
Waals multilayers.
