One of the more eerie parts of The Two Towers has Frodo and Sam following Gollum through a vast marshland shortly after they encounter him. In the midst of the fens they discover that the marshes are not normal at all, but house very disturbing images in their murky waters.
Just to the northwest of Mordor, between the Black Gate and the Emyn Muil, lies a large expanse of bogs and fens stretching for miles and miles across the barren land. Gollum seems to be very familiar with this area and describes its history to Frodo and Sam briefly.
The area was named the Dead Marshes and Sam quickly discovers why. At one point in their trek across the desolate land, small lights appear: little flickers of flame with no sign of their sources. These pop up all around the travelers and Gollum immediately warns the other two to not follow these lights, calling them “tricksy.”
Soon after, Sam stumbles and lands on his hands on the edge of a large mere. In the stagnant water he sees very clearly the image of a dead face and quickly pulls himself away. When he questions Gollum about it, he gets the following response:
“…You should not look in when the candles are lit…All dead, all rotten. Elves, and Men and Orcs. The Dead Marshes. There was a great battle long ago, yes…They fought on the plain for days and months at the Black Gates. But the Marshes have grown since then, swallowed up the graves; always creeping, creeping.”
To this, Sam replies with logic, saying that the bodies couldn’t have lain there for “an age and more,” asking if there was “some devilry hatched in the Dark Land.” Gollum replies:
“Who knows?.. You cannot reach them, you cannot touch them. We tried once, yes, precious. I tried once; but you cannot reach them. Only shapes to see, perhaps, not to touch.”
This description is about all there is in the legendarium about these Marshes, so far as you can trust Gollum’s account. Although not thoroughly explained in the film, it would appear as though there are no real bodies or corpses in the waters of the Marshes, just some image to trick unfortunate travels or something more sinister. The images are never mentioned nor explained again and neither are the lights. The battle Gollum alludes to is the Batttle of Dagorlad in which Sauron was first overthrown at the end of the Second Age.
Gollum leads Frodo and Sam out of the swamp over the course of several nights (refusing to travels while the Sun is out) and they have little other troubles besides the occasional fright of a flying Ring-wraith.