The Forest of Fangorn (somewhat speculative)¹

The deep, dark woodland that grew beneath the southern Misty Mountains, on the eastern flanks of that range. Millennia before the War of the Ring, these woods had spread across the western lands of Middle-earth, but the forestry of the Númenóreans had seen them dwindle during the Second Age, and by the end of the Third, the Forest of Fangorn was the last large part of that old forest to survive, though not the only one.

The forest took its name from the Elvish Fangorn, 'Treebeard', a reference to the old Ent who dwelt there. The Ents had once walked freely in the western lands, but with the failing of the forests, their range was diminished, so that by the time of the War of the Ring, they were only found in any numbers in the Forest of Fangorn. Though they had remained hidden in the Forest for centuries at this time, they emerged to play an important part in that War, overcoming Saruman's fortress at Isengard, and helping to destroy his troops.