WDS>Suppose we wanted to create an extremely high tower (think of it as a very long vertical rod) by combining the facts that (a) compressive strength of materials is large (b) "buckling" instabilities can be prevented by active control. Another idea is an extremely large-span arch (if a constant-diameter rod is bent into the shape of a "cycloid" "catenary"?--hgb curve, then the stresses in this arch are purely compressive). (Of course, it'd be a bummer if you had a power or software failure... or an airplane hit it...) [lots&lots of interesting analysis]<WDS John McCarthy once computed that an affordable column of tungsten (I forget his dimensions, something like a hundred meters by a few inches) would be a crust penetrator. Hans Moravec designed towers many miles high rigidified by a circulating stream of superconducting "niobium cannonballs". You could also build very large "vacuum balloons" thus stiffened. --rwg