A ground swell is a broad, deep undulation of the ocean caused by a distant storm. Powerful waves travel for great distances long after the originating tempest has been exhausted. Ships and shores are pulled and pushed by the effects of unseen storms.
The ground swell is a metaphor for the impermanence of the world. Forces both seen and unseen ensure that nothing in this world lasts forever. What feels solid and permanent is inevitable exposed to be ephemeral and temporary.
"Then I looked on all the works that my hands had done
And on the labor in which I had toiled;
And indeed all was vanity and grasping for the wind."
Standing on solid ground is an illusion. We've always been floating on an ocean.