"To hide something from a Grey Wizard, give them enough false clues to reach a wrong answer that makes them feel clever," he says with a smile. "To hide it from many, have enough wrong answers for them to argue over. Who says any of the access points are physical?"
"Basic logic," you retort. "The College has to be somewhere, so having at least some of the access points be physical is only sensible. Saves magical power and effort, and allows an access point in case of local magical collapse."
"The College has to be somewhere," he repeats, his smile widening. "A logical assumption." He places a hand on the wall and it ripples and melts away, revealing... nothing. A uniform field of grey on the other side, stretching as far as you can see in all directions. "The problem with logical assumption is that logic often has nothing to do with our profession."
He walks through the opening and without any visible trepidation you follow suit, walking atop an invisible floor. The wall heals below you and vanishes, enfolding the pair of you in an endless grey void. It's just as featureless to your Magesight as it is to your eyes. "Where are we?"
"To everyone else, the border between this world and the Aethyr is as thin as a dream. To those with the right attunement, the border can be a plane in its own right." He takes a seat on a chair you're certain was not there a moment ago. "It is the source of some of our most potent abilities. This is where objects enfolded within Substance of Shadow reside, this is the impossibly thin and thus infinitely sharp edge summoned by Penumbral Pendulum, this is where the Pit of Shades opens into, creating a wailing as the air itself is crushed. And through Teclis' techniques that I've only begun to scratch the surface of, it can be permanently expanded into a pocket big enough for a College."