  We explore the properties of atoms confined to the interstitial regions
within a carbon nanotube bundle. We find that He and Ne atoms are of ideal size
for physisorption interactions, so that their binding energies are much greater
there than on planar surfaces of any known material. Hence high density phases
exist at even small vapor pressure. There can result extraordinary anisotropic
liquids or crystalline phases, depending on the magnitude of the corrugation
within the interstitial channels.
