Ilmen
From Tolkien Gateway
Ilmen is a part of the atmosphere that surrounds the world of Arda before the cataclysm at the end of the Second Age. Ilmen forms the outer layer of normal air: unlike Vaiya it only exists above Arda itself. Below Ilmen is the inner air, Vista.
The Moon passes through Ilmen on its way around the world, plunging down the Chasm of Ilmen on its way back.
