OP is apparently looking for 1D structures:

1. On one hand, a 3+1D [Kerr black hole](https://en.wikipedia.org/wiki/Kerr_metric) has a 1D ring singularity.

2. On the other hand, an [event horizon](https://en.wikipedia.org/wiki/Event_horizon) has always [codimension](https://en.wikipedia.org/wiki/Codimension) 1 in spacetime. In 3+1D we speak of the event-horizon  as a _2D area,_ i.e. we are implicitly suppressing 1 dimension. According to that terminology the event-horizon becomes a _1D length_ in 2+1D.