In the Convoluted Universe books by Dolores Cannon, the Aliens said portals are a naturally occuring phenomenon on Earth. Entering a portal takes you to another part of our Universe, not another Dimension. Aliens can create Windows that can take them to another Dimension. A Window can also be used to just watch what is going on in another Dimension without being seen or heard. Beasts, monsters and sea serpents can go in and out of portals on Earth to and from another part of our Universe.