The Boundary

The Boundary was a portal on the shore of an unnamed planet in the far future, activated by the proximity of humanoid people. It worked as a gateway, a direct route into another random part of the universe. The Boundary patterns were said to never the same twice, so it was used by the humans during the late Cyber-Wars to escape the Cybermen. It was guarded by Ko Sharmus, who helped the fugitives to get through it.

Eventually, it worked as a gateway to the Gallifrey ravaged by the Master. The Thirteenth Doctor believed that the Master was responsible for this, though the Master refused to explain how he connected the Boundary to Gallifrey. This portal, or a similar one, is also what the Timeless Child went through to land in our universe. It was beneath the portal, on possibly the same unknown planet that Ko Sharmus was found on, that Tecteun found the child and took her home the Gallifrey. (TV: Ascension of the Cybermen, The Timeless Children)