  Moir\'e superlattices in transition metal dichalcogenide (TMD)
heterostructures can host novel correlated quantum phenomena due to the
interplay of narrow moir\'e flat bands and strong, long-range Coulomb
interactions1-5. However, microscopic knowledge of the atomically-reconstructed
moir\'e superlattice and resulting flat bands is still lacking, which is
critical for fundamental understanding and control of the correlated moir\'e
phenomena. Here we quantitatively study the moir\'e flat bands in
three-dimensional (3D) reconstructed WSe2/WS2 moir\'e superlattices by
comparing scanning tunneling spectroscopy (STS) of high quality exfoliated TMD
heterostructure devices with ab initio simulations of TMD moir\'e
superlattices. A strong 3D buckling reconstruction accompanied by large
in-plane strain redistribution is identified in our WSe2/WS2 moir\'e
heterostructures. STS imaging demonstrates that this results in a remarkably
narrow and highly localized K-point moir\'e flat band at the valence band edge
of the heterostructure. A series of moir\'e flat bands are observed at
different energies that exhibit varying degrees of localization. Our
observations contradict previous simplified theoretical models but agree
quantitatively with ab initio simulations that fully capture the 3D structural
reconstruction. Here the strain redistribution and 3D buckling dominate the
effective moir\'e potential and result in moir\'e flat bands at the Brillouin
zone K points.
