In the idealized mathematical model of a rotating black hole, the singularity has the shape of a ring.
This result comes from the Kerr solution, which describes the spacetime outside a rotating, uncharged black hole. Unlike the point-like singularity in the simplest nonrotating model, the Kerr singularity is represented mathematically by a ring lying in the black hole’s equatorial plane.
The ring is not a solid object that astronomers have photographed. It is a feature of a classical solution to Einstein’s equations, and the equations become singular there. A complete theory combining general relativity with quantum mechanics may change how such extreme regions are understood.
The ring singularity is also associated with unusual mathematical features in the idealized Kerr geometry, including paths that appear to pass through the ring into another region of the extended solution. These speculative extensions are not evidence that real black holes contain traversable portals.