Destiny Times Three by Fritz Leiber

But who will reveal to our waking ken The forms that swim and the shapes that creep Under the waters of sleep? _The Marshes of Glynn_, Sidney Lanier.

Like a dreamer who falls head-foremost for giddy miles and then is wafted to a stop as gently as a leaf, Thorn plunged down the main vertical levitator of the Opal Cross and swam out of it at ground level, before its descent into the half mile of basements. At this hour the great gravity-less tube was relatively empty, except for the ceaseless silent plunge and ascent of the graduated subtronic currents and the air they swept along. There were a few other down-and-up swimmers -- distant leaflike swirls of color afloat in the contracting white perspective of the tube -- but, like a dreamer, Thorn did not seem to take note of them.