a carbon nanotube
A carbon nanotube is a one-atom-thick sheet of carbon rolled into a seamless tube — only a few nanometers across, but it can be very long. Picture rolling chicken-wire into a straw, except the wire is made of individual carbon atoms locked in a hexagonal mesh.
Structurally it is a rolled-up sheet of graphene: the wall is carbon arranged in hexagons, every bond covalent. A single defect-free tube can reach a tensile strength near 100 GPa (many times that of steel) and a Young's modulus around 1 TPa, and — depending on the exact angle at which the sheet is rolled, its chirality — it behaves as either a metal or a semiconductor. Tubes come as single-wall or multi-wall (tubes nested inside tubes).
This matters because carbon nanotubes are among the strongest fibers known and can conduct electricity. But be honest: a single tube is astonishing, while a real rope or composite falls far short — defects, poor alignment, and weak load transfer between tubes (they slide past one another) drop the usable strength by orders of magnitude. Cost and getting them to disperse evenly remain hard problems.
Imagine taking a one-atom-thick sheet of carbon and rolling it into a tube 2 nm across but micrometers long. Every carbon is covalently bonded to three neighbors, so pulling on the tube pulls directly on those bonds: a single defect-free tube can reach tensile strengths near 100 GPa, roughly 50 times a strong steel, at about one-sixth the density.
A seamless rolled-up carbon sheet is one of the strongest fibers known — per tube.
The headline strength is for a perfect individual tube; in a real rope or composite, defects, misalignment, and weak tube-to-tube sliding drop the usable strength by orders of magnitude.