  The origin of our Galaxy's high-velocity clouds (HVCs) remains a mystery
after many decades of effort. In this paper, we use the TNG50 simulation of the
IllustrisTNG project to identify cool, dense clouds that match observations of
Galactic HVCs. We track these clouds back in time to determine their origin. We
find that only 17% of HVCs (by mass) can be tracked directly to the disk, and
21% to satellite accretion. The majority of HVCs (62%) arise from warm and hot
circumgalactic gas that cools through thermal instability. They then obtain
their anomalous velocities through interactions with the turbulent
circumgalactic medium. At TNG50 resolution, we do not see evidence for HVCs
forming out of very low metallicity intergalactic material. Instead, low
metallicity HVCs are most likely associated with satellite accretion. These
results suggest that Galactic HVCs are highly heterogeneous in their origin,
and can provide insight into the physical processes that shape the
circumgalactic medium such as disk outflows, satellite accretion, and thermal
instabilities.
