  We elucidate the microscopic quantum mechanism of superfluid $^4$He by
uncovering a novel characteristic of its many-body energy levels. At
temperature below the transition point, the system's low-lying levels exhibit a
fundamental grouping behavior, wherein each level belongs exclusively to a
single group. In a superflow state, the system establishes thermal equilibrium
with its surroundings on a group-specific basis. Specifically, the levels of a
selected group, initially occupied, become thermally populated, while the
remaining groups of levels stay vacant due to absence of transitions between
groups. The macroscopic properties of the system, such as its superflow
velocity and thermal energy density, are statistically determined by the
thermal distribution of the occupied group. Additionally, we infer that the
thermal energy of a superflow has an unusual relationship with flow velocity,
such that the larger the flow velocity, the smaller the thermal energy. This
relationship is responsible for a range of intriguing phenomena, including the
mechano-caloric effect and the fountain effect, which highlight a fundamental
coupling between the thermal motion and hydrodynamic motion of the
system.Furthermore, we present experimental evidence of a counterintuitive
self-heating effect in $^4$He superflows, confirming that a $^4$He superflow
carries significant thermal energy related to its velocity.
