The reason it's called the Haunted Marsh is because a countless number of cats have wandered to it and sooner than later found as skeletons. Most cats that go to it without entering it hear dead cat's voices-- although, that can't be proved because no ShadowClanner has actually decided to put themselves in that much danger to see. They don't want it as part of their territory because they fear that if they do the spirits of cats forced to forever swim in the murky waters will roam freely on the rest of their territory.