Frodo seemed the most weary of the three, and slow though they went, he often lagged. The hobbits soon found that what had looked like one vast fen was really an endless network of pools, and soft mires, and winding half-strangled water-courses. Among these a cunning eye and foot could thread a wandering path. Gollum certainly had that cunning, and needed all of it. His head on its long neck was ever turning this way and that, while he sniffed and muttered all the time to himself. Sometimes he would hold up his hand and halt them, while he went forward a little, crouching, testing the ground with fingers or toes, or merely listening with one ear pressed to the earth….
Presently it grew altogether dark: the air itself seemed black and heavy to breathe. When lights appeared Sam rubbed his eyes: he thought his head was going queer. He first saw one with the corner of his left eye, a wisp of pale sheen that faded away; but others appeared soon after: some like dimly shining smoke, some like misty flames flickering slowly above unseen candles; here and there they twisted like ghostly sheets unfurled by hidden hands. But neither of his companions spoke a word.
At last Sam could bear it no longer. ‘What’s all this, Gollum?’ he said in a whisper. ‘These lights? They’re all round us now. Are we trapped? Who are they?’
Gollum looked up. A dark water was before him, and he was crawling on the ground, this way and that, doubtful of the way. ‘Yes, they are all round us,’ he whispered. ‘The tricksy lights. Candles of corpses, yes, yes. Don’t you heed them! Don’t look! Don’t follow them! Where’s the master?’
— J.R.R. Tolkien, The Two Towers, “The Passage of the Marshes”