Annealing of ball-milled powders of graphite and boron nitride leads to the formation of nanotubes. Homogeneous and heterogeneous nucleation processes are active. Tip nucleation by metal impurities, and surface diffusion gives rise by catalytic capillary growth to 'bamboo' and larger 'skeletal' nanotubes with a pronounced endohedral crystallographic architecture comprising inverted and faceted cuneiform voids. Dominant surface diffusion drives the growth of the nanotube. The internal structure is due to a fluid-like capillarity of the precipitate-thermodynamics and the capillarity of solids-which shapes and reshapes during nanotube extrusion. (C) 2002 Elsevier Science B.V. All rights reserved.