Fountain of youth perhaps?
While strolling through the woods in his backyard, the OP stumbled across a pond that he had never seen before. Not only was it his first time laying eyes on the pond, but it was also the first pond that looked manmade that he’d ever seen. No body of water is perfectly circular, so what was he staring at?
It turns out that this is a settling pond, usually used to prevent erosion while also water moves downstream. At least, that’s what Google says. Maybe redirecting water from one source to another is supposed to stop it from breaking down rocks and other materials over time?