I am fascinated by the possibility of using fullerenes to build eternal structures. If not eternal, extremely long-lasting. Fullerenes already exist today. See?
Above are aggregated diamond nanorods (ADNRs). The name "hyperdiamond" recently appeared to describe this material.
ADNRs, a type of fullerene (any molecule made entirely out of carbon), is the hardest and least compressible known material. Its bulk modulus, meaning resistance to compression, is 491 gigapascals (GPa), beating diamond which is only about 445 GPa. For comparison, the bulk modulus of steel is 160 GPa, glass is 30 GPa, and bone is just 15 GPa.
What else? This black stuff:
Look how dark it is. Something made out of that would be hard to see at night. Also, its melting point would be several thousand degrees.
The image above shows one of the longest nanotube forests ever created. The nanotubes are about 8 mm long.