Abstract
Neutral hydrogen data is discussed for HVCs with velocities over the arbitrary value of 80 km/s. The results of recent high sensitivity surveys are reviewed, and a preliminary report of Arecibo observations with high angular and velocity resolution of a sample of HVCs with extreme negative velocities is presented. These surveys confirm the tendency of HVCs to cluster along narrow bands extending several degrees across the sky. The distribution of objects with velocity less than -200 km/s shows a clear hemispheric asymmetry. There is a preponderance of negative radial velocities, which implies a general inflow of H I towards the galactic disk. The contribution of the galactic rotation to the velocity distribution of the gas provides evidence for the existence of two large sub-samples of HVCs: a subgroup of relatively nearby features and a subgroup located in intergalactic space. Galactic fountain models of HVC origin are taxed to explain the properties of gas with extreme velocities.
