‘Yes, just think,’ the General said sardonically, ‘all you need is a four-ton megalith strapped to your back.’

A soundless explosion of blue light forced them to shield their eyes despite their sunglasses. When it cleared, the crackling bolts of lightning had gone and Glenning was standing near the bluestone, still wearing his RAF thermal flight suit. Hal could instantly see that something was wrong. The pilot was shaky on his feet, disoriented, his skin as pale as snow. But it was his eyes that would haunt Hal in the coming months. Buried in their depths was a terror so vast that it appeared to be consuming Glenning from within. This was a man who had personally seen the very depths of hell, someone who knew that his life was already over and that what was to come would be worse.

Glenning stared at them all through the Plexiglas window and desperation carved its way into his face. He reached out one arm, opened his mouth to plead, and then he simply… disintegrated. It was as though, Hal mused later, he had been a statue carved out of sand, collapsing under its own weight. A pile of grey dust was all that remained in the sterile atmosphere of the hall.

Hal couldn’t quite believe what he had seen. One second Glenning had been there, living and breathing and human, and then, unbelievably, he wasn’t. Hal’s mind rebelled at what his eyes were telling it. Gradually, though, his incredulity became dismay at the loss of a man in such an inhuman way.

‘Another failure,’ Kirkham said levelly, checking his monitors.

Jubilation drained from Catherine Manning’s face, leaving behind a sickened horror. ‘Oh my God,’ she said, her voice dry. ‘What’s happened to him?’

‘The only way we’re ever going to cross over is by defeating that fatal flaw in our make-up that self-destructs whenever we try to pass through the barrier,’ Kirkham continued, ignoring the question.

‘We know that some people can do it,’ the General said. ‘We need to find out what they’ve got that we haven’t.’