  A new group of kagome metals AV$_3$Sb$_5$ (A = K, Rb, Cs) exhibit a variety
of intertwined unconventional electronic phases, which emerge from a puzzling
charge density wave phase. Understanding of this parent charge order phase is
crucial for deciphering the entire phase diagram. However, the mechanism of the
charge density wave is still controversial, and its primary source of
fluctuations - the collective modes - have not been experimentally observed.
Here, we use ultrashort laser pulses to melt the charge order in CsV$_3$Sb$_5$
and record the resulting dynamics using femtosecond angle-resolved
photoemission. We resolve the melting time of the charge order and directly
observe its amplitude mode, imposing a fundamental limit for the fastest
possible lattice rearrangement time. These observations together with ab-initio
calculations provide clear evidence for a structural rather than electronic
mechanism of the charge density wave. Our findings pave the way for better
understanding of the unconventional phases hosted on the kagome lattice.
