Easy but not simple to open. Must be actively maintained as open, probably in some simple way, like by pumping sound or light into the event horizon. Let's say that the process involves micro black hole residue left over from the primorial universe, and that they are pretty much everywhere. So far my best idea is that someone was playing around with lasers and made a portal using an array of lasers at two different wavelengths such that the beams formed an overlapping region in the middle, where one wavelength activated the micro black holes when the beams were directed directly into one another. Let's say that different things can happen with different setups, such that sometimes, things go through in both the source and target medium at different strengths. Let's say that the discovery happened when they shined another laser through the portal device and noticed that it came out the other size at double strength because the laser on the far side was a combination of the laser on the nearside of each side of the rift. Obviously none of this makes much sense from a science perspective, but that's ok.

This was more productive than normal. I tried a strategy where I focused on ideas for a more specif aspect of the world. Also, I think the fact that this aspect relates to the origin of the rift helped a lot because then I wasn't working on the middle of a story to which I did not know the beginning, but did know the end, more or less.