"Trapped in the Sea of Stars" by Fritz Leiber is a neat little story in the highly original fantasy series about Fafhrd and the Gray Mouser. There is compelling action without a great deal of plot: Fafhrd and the Mouser are aboard a small fast sailing ship in the open sea, following a pair of ethereal sprites (evanescently hinted girl-shapes) when they come too close to the strong equatorial current of their world of Nehwon.
The standard world-theory held by its inhabitants is that Nehwon is a bubble of land and air floating within the universal ocean. Fafhrd the barbarian doesn't believe this, but his colleague-in-arms the Gray Mouser proves surprisingly perceptive in grasping the details of this cosmology — or surprisingly imaginative in inventing them. This is an idea which has been (so to speak) floated before, and not just on Nehwon:
Oh blindness to the future! kindly given,
That each may fill the circle marked by heaven:
Who sees with equal eye, as God of all,
A hero perish, or a sparrow fall,
Atoms or systems into ruin hurled,
And now a bubble burst, and now a world.
An Essay on Man, Epistle 1 (1733)
Throughout the Fafhrd and the Gray Mouser stories we hear mentions of the world-bubble theory. In "Trapped in the Sea of Stars", Leiber develops it in fine and subtle style; and it almost makes sense. Quite an imaginative construct! Read it quickly if you must for the adventure and the erotically teasing sprites, but then re-read it slowly for the portrait of the cosmos: sly, bemused, tantalizing.
They were lured on by a pair of shimmer-sprights, as they are called, a breed of will-o'-the-wisps which men deem infallible guides to lodgements of precious metals, if only one have a master hunter's patience and craft to track them down, by reason of which they are also called treasure-flies, silver-moths, and gold-bugs. ...
That morning as their treasure-sprights led them on, looking like rays of rainbow in the sun, the Mouser and Fafhrd were each lost in his secret thoughts of girls and gold, so that neither noted the subtle changes in the ocean surface ahead, from ripply to half smooth with odd little long lines of foam racing east.