  The newly discovered Kagome superconductors AV$_3$Sb$_5$ (A=K, Rb and Cs)
continue to bring surprises in generating unusual phenomena and physical
properties, including anomalous Hall effect, unconventional charge density
wave, electronic nematicity and time-reversal symmetry breaking. Here we report
an unexpected emergence of multiple flat bands in the AV$_3$Sb$_5$
superconductors. By performing high-resolution angle-resolved photoemission
(ARPES) measurements, we observed four branches of flat bands that span over
the entire momentum space. The appearance of the flat bands is not anticipated
from the band structure calculations and cannot be accounted for by the known
mechanisms of flat band generation. It is intimately related to the evolution
of van Hove singularities. It is for the first time to observe such emergence
of multiple flat bands in solid materials. Our findings provide new insights in
revealing the underlying mechanism that governs the unusual behaviors in the
Kagome superconductors. They also provide a new pathway in producing flat bands
and set a platform to study the flat bands related physics.
