Carbyne is a chain of carbon atoms linked either by alternate triple and single bonds or by consecutive double bonds. This material does not exist in nature (although astronomers believe they have detected its signature in interstellar space) but it can be synthesized (a couple of years ago researchers managed to make carbyne chains up to 44 atoms long in solution).
Researchers thought that it is a very unstable material (some chemists have calculates that two strands of carbyne coming into contact would simply explode). But now researchers calculated that Carbyne is about twice as stiff as the stiffest known materials today, and is it significantly stronger than diamond, carbon nanotubes and graphene.
The researchers also say that two carbyne chains will indeed explode on contact, but they will not contact each other at all because of an activation barrier.