There was concern early in the design stage: Allen predicted various ills, from rapidly deteriorating depth vision to catastrophic claustrophobia, psychosis to asphyxiation, loss of hearing, balance, and an inability to digest food. He insisted the crew needed access to the outside, a visual way out, proof of a world bigger than the interior, and Control considered breaking the seal and installing a true observation window.

In the end, they settled on this compromise: a false porthole permanently displaying a single live feed from one of the cameras mounted on the outside. The difference was marginal: the camera was the other side of the screen, 8 feet back. The transmission lag was microseconds. We really were seeing what was there. Most of the time you didn’t think about it – it was a convincing reproduction, we recognised the porthole unconsciously, and so far at least, the dire problems Allen had predicted hadn’t come to pass.

Sometimes, though, you looked at it and told yourself this wasn’t a real window, that you were actually staring at a massively fortified barrier. The effect was strange, as you struggled to hold both realities at the same time: the appearance of looking out and the fact of being entirely walled in.