Some have postulated that the Progs (the creators of Hollow) left their outside-in world when they realised they had lost control over the arrival of aliens from other planets.
It turned out that Hollow warps every portal in the universe.
It doesn’t matter whether a portal is naturally occurring or artificial, it will point to Hollow. If someone enters a portal on a planet outside of Hollow they will always end up in Hollow due to Hollow’s interstitial magnetism.
And there is no way to leave. A portal traveller can’t just turn around and go back through a portal they have just stepped out of because it isn’t there. Hollow itself is the exit point. Seeing as they are already inside Hollow they can’t just step inside it again as it were.
The only way of departing Hollow is to use the same method as the Progs: Interstitial Travel. Unfortunately the only people who have ever invented the Interstitial Travel Device (ITD) are the Progs. (And a human called Montgomery-Jones but he did it accidentally).
The Progs departed using their ITDs but left some artefacts behind like the buildings set into the mountains of Skarnelm. They also left several types of devices. Some of these are known – like the nano machines that clean up most virulent bacteria and the semi-real devices, nicknamed “mindlearns”, that enable beings to communicate with one another – but it is suspected there are other artefacts and devices that haven’t been found yet.
Mindlearns can be obtained from thousands of points around Hollow. A bit like vending machines, they provide mindlearns when they are approached by any intelligent being. Attempts to open the machines to find out how they work have always met with failure.
The eminent scientist Bakalwe (deceased) worked out a way to transmit a mindlearn from a nearby node to an intelligent being even though that being was not near a node at the time. Ranthar and Lungwill used this method to install a mindlearn in Drome when he encountered their remotely controlled debris creature.