WHEN I SPOT a potential swimming hole during a summer road trip, I morph into Pavlov's dog. Except I don't waste valuable time drooling; I just shed. My clothes, that is, after ditching the car and before jumping in. One caveat: when contemplating a hot spring, I make a point of dipping a foot in first to gauge the cauldron's kick.

Unless, that is, I'm in Yellowstone National Park, where I wouldn't advise either tentative toe test or plunge. Spanning the northwest corner of Wyoming, plus slivers of Montana and Idaho, the park seethes with hydrothermal activity, and blistering water temperatures mean the hot springs are better suited to boiling you alive than offering a relaxing soak.

With that in mind, it probably won't come as a surprise , if you happen to be visiting the park this summer, to find that swimming areas aren't labeled on any park maps. The park prohibits dips into thermal pools and source streams. However, don't assume you can't swim. You won't find any neon signs marking the routes, but two out-of-the-way settings lure summertime visitors who've eyed the sulfur-belching mud pots and still want to get wet.

Along the Firehole River just south of Madison, a one-way road winds past soaring volcanic rock cliffs. Ringing the canyon are elegant skeletons of ravaged lodgepole pines, evidence of the 1988 fires, as well as a carpet of emerging green. Past a 40-foot waterfall, the river funnels through a narrow chasm and creates a series of deep blue pools.

Free amusement park rides exist in nature, if you know where to find them. Starting from a small up-current basin, a jet of white water shoots to the next pool, and you can ride it like a lazy log flume. After the rush dies down, you'll float by a small cave to a large and placid swimming hole. On a hot August afternoon last summer, just a day after snow flurries dusted Old Faithful, a dozen blissful swimmers took turns body surfing the rapids and drifting face up in the blazing sunshine. Possibly the best detail is the hot spring that dumps into the river upstream, warming up an otherwise icy experience.

Situated around an enormous volcanic caldera, Yellowstone's geyser-saturated expanse shelters eye-popping herds of bison and elk. They seem to graze everywhere, stalked by a corresponding number of telephoto lens-laden tourists. Park literature handed out at the entrance somewhat hilariously illustrates the tendency of the bison, so cute and cuddly-looking, to gore people who get too close. The warning seems to have a short shelf life. On one road we gawked not at the nearby bison but at the monster RV ambling off the road in pursuit of them. The camper inched closer and closer to a mother and her calf, finally balancing on three wheels at the precipice of a ditch.

Fences and walkways ensure that sightseers don't stray too close to the hissing fumaroles or goopy paint pots, but the iconic bison aren't always so fortunate. While warming themselves in wintertime, they occasionally fall through the brittle edges of thermal springs. Sometimes they stew for a while and then shoot out like cannonballs, or hot meals for ecstatic wolves and bears.

I nearly shared the same fate. While strolling the narrow wooden boardwalks in the Norris Geyser Basin area, I entertained no crazy notions about poking body parts into the bubbling, algae-streaked waters. My suicidal hiking boots had other ideas. The lace from one shoe snagged onto a hook from the other, and I started to fly through the air. Thankfully the hook snapped off, and I was spared the irony of death by soup, my favorite food.

On a wide plain between low scrubby hills, wafting clouds of steam signal hot spring heaven: a soaking spot called Boiling River. Formed by the intersection of a frigid roaring river and a smoldering channel, it is aptly located on the 45th parallel, halfway between the equator and the North Pole. Bathers position themselves in a curious human barrier down the confluence of boiling and freezing, constructing rock walls to control the water flow. Simultaneously prickly and pleasurable, it is an odd sensation, like sitting between blasting hot and cold faucets. Lean too far to the left and you scald, too far to the right and your lips turn blue.

Climbing into the river, I raced to reach the mixing point a few yards in, detecting a smirky schadenfreude from those already comfortably nestled there. After some futile maneuvering to secure a perfect spot, I realized that the temperature changed every time someone moved. Eventually I found a tranquil whirlpool near the shore and let myself relax. As I looked up through the steam, the big sky seemed larger than life, and I dreamed of a soggy but endless summer.

Beth Kohn is a freelance writer who lives in San Francisco. Email her here.

Trip planner

The park Admission is $20 a car. (307) 344-7381, www.nps.gov/yell.

Getting wet The Firehole swimming area is in the central western area. From Madison go south for less than a mile to Firehole Canyon Drive. Drive two miles in and park near the restrooms.

Near the northern park entrance, Boiling River hot springs sit next to the Wyoming-Montana border on Highway 89. Park at the lot near the 45th Parallel Bridge between Mammoth Hot Springs and Gardiner and follow the trail about a half mile to the signed Gardner River access.