  We use spectroscopic data from HST and FUSE to study the wide range of
ionization states of the "highly ionized high-velocity clouds". Studied
extensively in OVI absorption, these clouds are usually assumed to be infalling
gas in the Galactic halo at distances less than 50 kpc. An alternative model
attributes the OVI (and OVII X-ray absorption) to cosmological structures of
low-density, shock-heated intergalactic gas, distributed over 1-3 Mpc
surrounding the Milky Way. The latter interpretation is unlikely, owing to the
enormous required mass of gas (4x10^12 M_solar). Our detection, in 9 of 12
sight lines, of low ionization stages (CII/III/IV; SiII/III/IV) at similar high
velocities as OVI requires gas densities far above that (n_H=5x10^-6 cm^-3)
associated with the warm-hot intergalactic medium (WHIM). These HVCs are
probably cooling, multiphase gas in the Galactic halo, bow-shocks and
interfaces between clouds falling through a hot, rotating gaseous halo. The
velocity segregation of these HVCs in Galactic coordinates is consistent with a
pattern in which infalling clouds reflect the sense of Galactic rotation, with
peculiar velocities superposed.
