  Understanding the complexities of electronic and magnetic ground states in
solids is one of the main goals of solid-state physics. Materials with the
canonical ThCr$_2$Si$_2$-type structure have proved particularly fruitful in
this regards, as they exhibit a wide range of technologically advantageous
physical properties described by "many-body physics," including
high-temperature superconductivity and heavy fermion behavior. Here, using
high-resolution synchrotron X-ray diffraction and time-of-flight neutron
scattering, we show that the isostructural mixed valence compound,
KNi$_2$S$_2$, displays a number of highly unusual structural transitions, most
notably the presence of charge density wave fluctuations that disappear on
cooling. This behavior occurs without magnetic or charge order, in contrast to
expectations based on all other known materials. Furthermore, the
low-temperature electronic state of KNi$_2$S$_2$ is found to exhibit many
characteristics of heavy-fermion behavior, including a heavy electron state
($m^*/m_e \sim$ 24), with a negative coefficient of thermal expansion, and
superconductivity below $T_c$ = 0.46(2) K. In the potassium nickel sulfide,
these behaviors arise in the absence of localized magnetism, and instead appear
to originate in proximity to charge order.
