[Jinora looks at him, puzzled. She thought everyone knew, but maybe he came from somewhere remote that doesn't get news very often.]

Avatar Korra left the Spirit Portals open after Harmonic Convergence. That meant the spiritual world and the physical world merged, since nothing was sealing the spiritual world away anymore.

[She looks around at the vines.] They're trying to make their homes in our world. It's a shame about the buildings, but it's their home now too.