  The recently discovered nonlinear Hall effect (NHE) in a few non-interacting
systems provides a novel mechanism to generate second harmonic electrical Hall
signals under time-reversal-symmetric conditions. Here, we introduce a new
approach to engineering NHE by using twisted moir\'e structures. We find that
the twisted WSe$_2$ bilayer exhibits a NHE when tuning the Fermi level to the
moir\'e flat bands. Near half-filling of the first moir\'e band, the nonlinear
Hall signal shows a sharp peak with the generation efficiency at least two
orders of magnitude larger than those in previous experiments. We propose that
the giant NHE and diverging generation efficiency originate from a
mass-diverging type continuous Mott transition, which is evidenced by
resistivity measurements. This work demonstrates not only how interaction
effects can couple to Berry curvature dipoles to produce novel quantum
phenomena, but also what NHE measurements can provide for developing a new tool
to study the quantum criticality.
