I FIRST HEARD ABOUT the underground world of Florida golf-course fishing three years ago, when a buddy sent me a photo of a huge largemouth bass he'd caught in a pond at his in-laws' retirement home near Sarasota. The picture was a classic "grip and grin," in which the triumphant angler holds up his quarry and smiles like an idiot. The three-inch fly looked like a tiny crumb in the fat fish's cavernous maw. "Biggest bass I've ever caught," he said. Right then I decided that whenever I visited my folks, who live in a golfy gated community in Fort Myers, I'd set the six-iron aside for a six-weight fly rod.

Urban anglers have the housing boom to thank for this fishery. Builders decided they had to plop their stucco ranch homes around golf courses in order to sell them, a pricey development scheme that ultimately proved unwise. By the time the housing bubble burst, southwest Florida was littered with a surplus of golf courses (and, naturally, golf-course ponds), some still in business, some gasping, some dead. Most contain plenty of fish, and the largemouth isn't the only species you're likely to find.

At a North Fort Myers tackle shop I visited during a recent trip, the man behind the counter told me never to be surprised by the watery creatures I might run into on a golf course. "It isn't just bass in there," he said, pointing to a wall-mounted tarpon hanging in a frozen leap. "Some of them are almost too big for those little ponds. I've hooked ones up to 70 pounds."

"You'll probably have better fishing luck going to some golf-course pond than going up to Lake Okeechobee," concurred Mark Ward, a Naples-based guide who specializes in prized species like snook, redfish, and tarpon. In addition to bass, Ward has seen 30-pound tarpon roll in streetside ponds more than a mile inland from the Gulf of Mexico. He once caught a few Mayan cichlids—a colorful panfish native to Mexico and Central America—in a small water hazard. "How they got in there," he says, "I don't know."

There are three leading theories as to how the ponds get fishy. The most obvious is that they're often connected to canals or natural bodies of water that already hold bass. The second possible way is via birds, which carry eggs on their feet from one pond to the next. The third is a shadowy notion I like to call Enterprising Angler Theory, the idea being that fishermen quietly stock the ponds because they know the fish will survive and grow, unmolested by people who, unlike them, aren't into skulking around.

How the fish get in the ponds is ultimately irrelevant. What matters is that they're there, which to me is an inherently thrilling proposition. I live in Washington, D.C., where the local fishing opportunities are just fine, thanks. But they aren't Florida fine, and as a fisherman I get worked up simply by entering Sunshine State airspace. Throw in the golf-course equation—the fact that I can drag one of my poorly dressed flies through a small, confined body of water and come away with anything from lunker bass to thrashing snook—and suddenly I'm convulsing with excitement.

Alas, it's not the easiest subculture to crack. Every Florida fisherman worth his weight in plastic worms seems to know about it, but the topic is hush-hush: no chat rooms, formal clubs, or governing bodies. When aficionados discover productive ponds, they squeeze them like state secrets. I learned this the first time I tried to pump a fly-shop owner for some promising golf-course leads. He refused to cough up a single course name, though he did offer this bit of self-serving advice: "Don't piss off the golfers. You'll ruin it for all of us."

As indicated by the paranoia, the legality of this sport varies from course to course, especially since Florida retirement communities regard themselves as something like sovereign nations. A few wise rules to follow: Always practice catch-and-release; obey the posted NO FISHINGu signs; fish until you're told not to, then buzz off for good; and dress nice. Khakis and a golf shirt will help you blend in. And if you're caught, you're more likely to be treated like some resident's preppie nephew than a random scumbag begging for prosecution.