Hearing the ker-plunk of a pond apple …
Into the center of a dome is a rite of passage in the swamp.

It also indicates two things:
Summer is advancing (thus it falling) and water is deep (thus the ker-plunk).
And how do you measure the worth of a pond apple?

What happens next after the ker-plunk?
The pond apple floats!
And starts to drift.
In the direction of sheet flow (until a log snags on a log).
Pond apples rarely drift far from the tree they fell.