There was no movement of air down under the forest-roof, and it was everlastingly still and dark and stuffy. Even the dwarves felt it, who were used to tunnelling, and lived at times for long whiles without the light of the sun; but the hobbit, who liked holes to make a house in but not to spend summer days in, felt he was being slowly suffocated. (Hobbit 8)

‘Yes, it is all very dim, and stuffy, in here,’ said Pippin. ‘It reminds me, somehow, of the old room in the Great Place of the Tooks away back in the Smials at Tuckborough: a huge place, where the furniture has never been moved or changed for generations. They say the Old Took lived in it year after year, while he and the room got older and shabbier together—and it has never changed since he died, a century ago. ...But that is nothing to the old feeling of this wood. Look at all those weeping, trailing, beards and whiskers of lichen! And most of the trees seem to be half covered with ragged dry leaves that have never fallen. Untidy. I can’t imagine what spring would look like here, if it ever comes; still less a spring-cleaning.’
‘But the Sun at any rate must peep in sometimes.’ said Merry. ‘It does not look or feel at all like Bilbo’s description of Mirkwood. That was all dark and black, and the home of dark black things. This is just dim, and frightfully tree-ish.' (LotR III.4)