But now the drift on the still water, Mysterious, beautiful- Explain

In " The Wild Swans At Coole" by W.B.Yeats 'they' refers to the wild swans which are now floating silently on this quiet, shallow part of the lake where there is no current. The Coole lake gradually gets engulfed with darkness as the evening sets in. This presents a very beautiful sight which suggests to the poet something strange and unspeakable. In the darkness the swans appear partially visible and seem to be be strange creatures coming from some unknown part of the world and drifting on the still water of the lake. To describe this queer look of the swans the poet called them mysterious. Additionally the vagueness of their activities in the dark also make them mysterious.