[Keeper of Dol Guldur]

I always wondered whether that poem was describing the gangrenous spirits who dwelt in the Dead Marshes.

I think it was the 'single candle lit' part that reminded me of the flickering candles in the marshes.

Of course, I always took the weird lights the hobbits and Gollum saw as Will o' the Wisps.

The Will o' the Wisp is the most common name given to the mysterious lights that were said to lead travellers from the well-trodden paths into treacherous marshes. The tradition exists with slight variation throughout Britain, the lights often bearing a regional name.

That's out of an article explaining Will o' the Wisps. It later references them as being malevolent spirits of dead, or non-human intelligence. They liked luring unwary travellers into dangerous situations.

In many places the Will o' the Wisp were associated with spirits of the dead who could not enter either heaven or hell, malignantly wandering the earth leading foolish travellers astray.

I don't know if the souls of all the elves who fell in the Last Alliance lingered, festered and became vile, or if spirits of Mordor inhabited the corpses and the marsh, but clearly whatever the spirits were, they weren't resting and relaxing in the Halls of Mandos.

More mundane explanations for the Will o' the Wisp come in the form of marsh gasses - natural methane - formed from rotting vegetation. The gas was thought to sometimes ignite spontaneously forming standing flames over boggy ground.

Which is totally what the movie depicted the marsh lights as.

It's interesting to note that another name for the Will o' the Wisp is this;

Corpse candles - related to graveyards and funeral processions.

Anyway, back on the subject of the Mewlips.

Clearly quicksand, slime, and dark waters are what the prey of the Mewlips fall into, and they see the grim faces of the Mewlips looking at them.

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.

A wisp of pale sheen (Will o' the Wisp, definitely). Some were like dimly shining smoke, some like misty flames.

One of the first questions Sam asked Gollum was; "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 tricksey lights. Candles of corpses, yes, yes. Don't you heed them! Don't look! Don't follow them! Where's the master?'

Check this out;

Sam looked back and found that Frodo had lagged again. He could not see him. He went some paces back into the darkness, not daring to move far, or to call in more than a hoarse whisper. Suddenly he stumbled against Frodo, who was standing lost in thought, looking at the pale lights. His hands hung stiff at his sides; water and slime were dripping from them.

A lot of talk of darkness, and slime.

'Come, Mr. Frodo!' said Sam. 'Don't look at them! Gollum says we mustn't. Let's keep up with him and get out of this cursed place as quick as we can - if we can!'

And then, a few sentences later;

He fell and came heavily on his hands, which sank deep into the sticky ooze, so that his face was brought close to the surface of the dark mere. There was a faint hiss, a noisome smell went up, the lights flickered and danced and swirled. For a moment the water below him looked like some window, glazed with grimy glass, through which he was peering.

And that is when he saw the face of a dead thing staring back at him.

He sunk a little, and looked at the grim dead face of an evil spirit, which seemed to live under a little glazed, dirty glassy window beneath the water.

Sounds like the Mewlips to me. Tolkien mentioned noisome smell, and the Mewlip poem mentioned noisome waters, and;

The cellars where the Mewlips sit
Are deep and dank and cold
With single sickly candle lit;
And there they count their gold.

Their walls are wet, their ceilings drip;
Their feet upon the floor
Go softly with a squish-flap-flip,
As they sidle to the door.

Naturally gives the idea of a creature living UNDER very wet ground, possibly even under the marshes. It's entirely possible, that the bodies of many of the elves and men inhabiting the Dead Marshes were in their own Barrows. So in that way, these would be very 'wight-like' creatures. Swamp Wights. But a Barrow which has been overtaken by the cursed swamp would be a lot like a cellar which was literally under the swamp.

As for droopy willow and gorcrows, it mentions them being nearby, but I'd guess that since it talks about a rotting river strand, it could mean the basin beneath Rauros, where Anduin becomes a criss-cross of marsh delta, because those wetlands spread all the way to the Dead Marshes, with nothing separating them other than the desolate Noman Lands.

As for the Merlock Mountains, I have no idea.

But willow trees are grey, and since we know a lot of willows were found hedging Entwash, and on rivers in Rohan, it seems likely that there would be the occasional rotting willow tree within a short distance of the Dead Marshes, in Nindalf.

Again, the last stanza mentions the Merlock Mountains, which could be the Misty Mountains, but in all seriousness could be any other mountains. But that 'long and lonely road' could refer to Sauron's causeway, which heads due north just east of the Dead Marshes, across Dagorlad, north through the Brown Lands and around up to Dol Goldur, but also likely splits off and heads north into Wilderland.

Through 'spider-shadows' could definitely be referencing Mirkwood. The Marsh of Tode could be ... the hard to travel marshes just east of Mirkwood on the River Running and the Forest River, they could be the Nindalf, or even the Dead Marshes themselves.

Through the wood of hanging trees, could definitely be referencing Fangorn.

I'm not sure about 'gallows-weed'. It seems like it could be an old fashioned term for some variety of grass growing in a swamp or scrubland, or it could be something Tolkien invented just because. Probably the old fashioned term is the more likely description.

That is where you'll find the Mewlips.

But could this very spot be the Mere of Dead Faces? That inky black pool of water in the very center of wight activity in the haunted marsh?

If you cross-reference that with Bilbo's journey, you can see the idea;

Just over the mountains, a long road takes you under the eaves of Mirkwood, down over lonely lands, to a distant marsh, where these fell creatures await in the deepest, darkest sections of the swamps.

Of course, Smeagol mentions that he has tried to reach them, tried to touch them, but they seem to be "only shapes to see, perhaps, not to touch."

Having attempted it, he may have more knowledge about their abilities than Hobbit poems, and discovered in his curiosity that they aren't out to collect bones, they are just trying to trick travellers into coming to a bad end.

Of course, once the traveller is dead, they may take his bones.

'Very carefully! Or hobbits go down to join the Dead ones and light little candles.'

So ... Gollum believed that the swamp wights indeed could have taken Frodo and Sam. How he discovered this, is a mystery. It's clear that Sam thought he had an idea of how.

So were the Mewlips the wights inhabiting the Dead Marsh?

I think it's pretty likely.