We report a novel crystalline carbon-cage structure synthesized from laser-driven shock wave loading of a graphite-copper mixture to about 14± 2 GPa and 1000± 200 K. Quite unexpectedly, it can be structurally related to an extremely compressed three-dimensional C 60 polymer with random displacement of C atoms around average positions equivalent to those of distorted C 60 cages. Thus, the present carbon-cage structure represents a structural crossing point between graphite interlayer bridging and C 60 polymerization as the two ways of forming diamond from two-dimensional and molecular carbon. Understanding the graphite-diamond transition mechanisms including transitory or intermediate phases may open windows for a more efficient synthesis of diamond. The graphite-diamond transition is also interesting by itself as a paradigm of reconstructive transitions involving changes in coordination, bonding and dimensionality, and for its possible relations with other transition routes to diamond from, for instance, C 60 polymers. However, the detailed mechanisms still remain unsettled despite considerable experimental and theoretical efforts motivated by the technological and scientific significance.Carbon is a complicated system 1 rich in polymorphs due to its ability to form sp-, sp 2 -, and sp 3 -hybridized C-C bonds including molecular phases such as buckyballs and buckytubes, the two-dimensional ͑2D͒ graphite, and threedimensional ͑3D͒ tetrahedral network of diamond. C 60 polymers, 2-5 carbyne, 6 and hypothetical cage structures such as carbon zeolites 7 establish intermediate phases intriguing for their structures and properties. The phase diagram and other high-pressure properties of C 60 have been investigated extensively using static techniques. 4,[8][9][10][11][12][13][14][15] The transitions of graphite and buckyballs to diamond involve major reconstruction of bonding and coordination, and thus are loadingrate-dependent processes. Dynamic experiments such as shock waves may be advantageous for revealing the transition mechanisms by metastable recovery of transient structures formed upon ultrafast loading. We have demonstrated for silica ͑another paradigm system͒ that a transient structure intermediate between tetrahedrally and octahedrally coordinated phases forms upon shock wave loading of quartz and coesite. 16 Diamond was synthesized by conventional shock wave ͑explosives or gas guns͒ loading of graphite alone or graphite-copper powder mixture to about 20 GPa and above. [17][18][19][20] Recent explosive-driven shock wave loading of pyrolytic graphite to about 15 GPa yielded several carbon allotropes including carbyne, a diamondlike phase and possibly fullerene. 21 It is worth noting that C 60 fullerene synthesis was pioneered by vaporizing graphite using lasers. 22 Naturally occurring fullerenes in meteorites and sediments were inferred to have formed during terrestrial impact. 23,24 Conventional shock wave loading of C 60 fullerene yielded amorphous diamond. 25 The differences in heating and strain rates, po...