And have it lurch out of the water to eat you in response.
/ kidding.
It’s probably a spring of some sort.
On the Neversink river near Huguenot NY, there is a sand boil in the river at the shore.
You take a seat, cast out your fishing line, look down and go “WTF?!”
There, next to your boots at the water edge, the sand looks possessed due to a spring bubbling up artesian style, but not forceful enough to fountain.