An immensely dense filament no wider than a proton but with the pull of a black hole, a fragment of which draws a shoal of two-dimensional lifeforms and the Enterprise-D with them.
A cosmic string fragment is the destination in the Next Generation episode The Loss. The Enterprise becomes caught in a group of two-dimensional creatures moving through space and cannot break out with engines or tractor beams.
Data determines that the beings are heading for a cosmic string, which emits a characteristic set of subspace frequencies as atomic particles decay along its length. The ship will be torn apart if it is carried all the way in.
Deanna Troi, who has lost her empathic sense because of the creatures' presence, reasons that they follow the string as a moth follows a flame. The deflector dish is used to imitate its vibrations behind the ship; the creatures hesitate, the Enterprise slips free, and Troi's abilities return.