String! I definitely wasn’t expecting to see this - it startled me when I first came across it, partly because it was looping up dramatically into midair before it sagged, as I watched, under the glare of the microscope’s electron beam. It came to rest draped over one of my other structures (a structure that came out fine, other than now being covered in string). I traced the string back and found that it had peeled off the top of a different structure, one that was etched too thin. The shredded edge would be the semiconductor laser material that these long columned walls are made of. The whole string’s about 100 nanometers in diameter - a thousand times thinner than a human hair.