It turns 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 – like a super-sized portal. Seeing as the traveller is 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 nodes around Hollow. A bit like philanthropic vending machines, nodes provide mindlearns when they are touched by an intelligent being. Attempts to cut open nodes to find out how they work have always met with failure.
The eminent scientist Horona Bakalwe (deceased) worked out a way to transmit mindlearns from nearby nodes to intelligent beings using an artificial creature as a conduit. Ranthar and Lungwil used this method to install a mindlearn in Drome when he encountered their remotely controlled debris creature.