Convulsions rocked the man's body. He glanced pleadingly up at Sean, shook his head, then his eyes rolled around and his mouth gaped open to give a low moan and a sound like a distant hissing. Sean could only watch in horror as something black and slimy wriggled out of his mouth; it slid out, then fell with a splash into the foul mess the man had just disgorged.
He rose to his feet, staring at Sean, and said: 'The . . . the centre . . .' before falling backwards into the raging water with a huge splash.
Sean was all set to rush to the water's edge to try and help the man out – but then he noticed the black slug-like thing move. In two minds, he glanced at the ferocious torrent and realized that he couldn't have saved the man anyway – even if there had been any life left in him. He looked down again – and screamed as the black thing started sliding, snake-like, towards him . . .