Ilmen was a part of the atmosphere that surrounds the world of Arda before the cataclysm at the end of the Second Age. Ilmen formed the outer layer of normal air: unlike Vaiya it only existed above Arda itself. Below Ilmen was the inner air, Vista. Tolkien likely derived its name from ilma, the Finnish word for air. The stars are found in this region. The Moon passed through Ilmen on its way around the world, plunging down the Chasm of Ilmen on its way back.