The Dead Marshes:

The Dead Marshes are to the north-west of the Dagorlad. They gained this name after years of slow creeping over the edges of the graves on the Dagorlad.
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 watercourses. (LOTR.651)

Somehow, the faces of the Dead, could be seen especially at night, an Age later:
They lie in all the pools, pale faces, deep deep under the dark water. I saw them: grim faces and evil, and noble faces and sad. Many faces proud and fair, and weeds in their silver hair. But all foul, all rotting, all dead. A fell light is in them. (LOTR.653)
Nobody knows by what dark magic these faces are still visible.