  We find that in ultraclean heavy-fermion superconductor URu$_2$Si$_2$
($T_{c0}=1.45$ K) a distinct flux line lattice melting transition with
outstanding characters occurs well below the mean-field upper critical fields.
We show that a very small number of carriers with heavy mass in this system
results in exceptionally large thermal fluctuations even at subkelvin
temperatures, which are witnessed by a sizable region of the flux line liquid
phase. The uniqueness is further highlighted by an enhancement of the
quasiparticle mean free path below the melting transition, implying a possible
formation of a quasiparticle Bloch state in the periodic flux line lattice.
