Experimental evidence is presented to show that the small penetrating component observed in the range distribution of keV projectiles in tungsten single crystals is caused by a diffusion process and not, as previously believed, by a "super-range" phenomenon. We suggest that a few of the injected atoms, probably the most perfectly channeled ones, come to rest interstitially. These interstitial atoms then diffuse rapidly, even at room temperature, until trapped by some defect or impurity atom in the lattice. The concentration of traps required to fit the experimental data is 1015-1016 per cm3. Various factors affecting such an interstitial diffusion mechanism are discussed.