One will not be seen here (4,3)
I believe the answer is:
thin air
But I can't explain it at all! Could you help me understand?
(Other definitions for thin air that I've seen before include "Things seem to vanish into this" , "Vanish into . . . . . . ., say high on Everest" , "To vanish into this is to become invisible" , "Very little to inspire" , "Vanish completely as into . . . . . . ., on high mountain?" .)