OLALLIE LAKE SCENIC AREA — We’d been trekking through the forest for half a mile, navigating by landmarks and occasional glances at the map, when a small body of water appeared through the trees.

“Ah-ha!” said Michael Donnelly, who was leading our trio. “I think this is ‘Fish-Shaped Pond.’”

We stepped through a grove of pines, sweet-smelling in the sunlight, to discover a lake shaped like a perfect goldfish cracker. Off-trail and deep in the forest, I took a moment to appreciate the cliff-lined shore and groves of blue huckleberries before taking a deep breath and diving into the clear, blue-green waters of the day’s second successful “lake bag.”

Not a bad way to spend the afternoon.

The sport goes by the name “lake bagging,” a quasi-competitive activity concocted by Donnelly and a few friends during the mid-1980s. The perfect pastime in Cascade Mountains during summer, the goal of lake bagging is visiting (and swimming in) as many mountain lakes as possible during the year’s hottest months.

There is no official committee of the lake-bagging society — or any grand commissioner to enforce official regulations — but there are a few rules to guide those exploring the countless blue dots scattered across Forest Service maps, both on the trail and off.

To wit:

1) Every named lake counts as one lake bag. Unnamed lakes, ponds or pools can count as well, but only if they’re deep enough to go over your head.

2) To achieve a “full bag” that can be added to your total, you must totally immerse yourself in the water (go for a swim, and dunk your head).

3) Pack out any garbage you find at the lake, and leave no trace.

4) If you’re going to swim in your birthday suit — in that most unabashed of fashions — make sure there’s nobody around who might be offended by a naked buttock or two.

Beyond that, the only real guidance is to seek out lake-rich areas in Oregon, usually the Cascades, and follow Donnelly’s example of simply going out and finding them.

“We started really checking out all the lakes, using a map and compass, and decided to have a competition to see who could swim in the most lakes in one year,” said Donnelly, who lives in Salem and is former vice-president of the Oregon Natural Resources Council, along with co-founder of the Friends of Opal Creek and Breitenbush Cascades. “The most anyone has reported bagging in a single season is Chad Frazier’s 58. I’d guess there are about 30 of us self-identified lake-baggers these days, and we’re pretty much on our honor.

“The ‘season’ is usually around eight weeks and begins when the lakes get around 70 degrees. We all keep threatening to buy wet suits to extend the season, but so far, no one has.”

The trick is finding a target-rich environment. And in that sense, few places are better than Olallie Lake Scenic Area.

Located northeast of Detroit off Breitenbush Highway 46, the Olallie backcounty is home to roughly 200 lakes both named and unnamed, on trail and off. The lakes sit around 5,000 feet, which means they’re usually open for hiking by mid-June and have warmed to a comfortable temperature by mid-July.

Lake Bagging is an oddball sport that probably won’t be featured on ESPN anytime soon, but the combination of navigating, hiking and swimming still makes a near-perfect way to enjoy the mountains during the year’s hottest months.