I’m having a surreal moment. I’m standing, covered head to feet in slick, graphite-colored volcanic mud, when suddenly the deep guttural honk of a howler monkey reverberates almost directly overhead. I walk gingerly up the slatted wooden walkway that traverses the boiling hot river, craning my neck for a glimpse of the monkeys and at the same time trying not to burn the soles of my feet on hot spots of volcanic steam puffing up from below.

This unusual experience is taking place at Borinquen Mountain Ecothermae Resort, high up on the slope of the Rincon de la Vieja volcano in Guanacaste, Costa Rica’s northernmost province. The border with Nicaragua is less than an hour away.

I’m staying at the resort, which has luxurious casita-style accommodations that run several hundred dollars a night. But visitors can have the same experience (albeit, truncated) by paying just $80 each for a day pass that includes a horseback ride to a stunning waterfall, a zipline adventure featuring cables criss-crossing the river gorge, and an afternoon at the hot springs.  Talk about deal of the day. If you have a car you can drive yourself or, for another $50 each, a minivan will pick you up at your hotel.

Borinquen isn’t new, it opened about five years ago, but it’s not heavily marketed and hasn’t made any of the major travel magazines. Now word is spreading among healthy types and spa-seekers because of its novelty. A spa resort that’s constructed directly over the boiling, bubbling fumaroles of an active volcano; that’s something you don’t see everyday. (Imagine if the discoverer of Yellowstone had, instead of designating it a national park, chosen to slap a spa on top of the famous geysers and mud pots.)

No question an afternoon at Borinquen is a unique experience. The spa’s steam room? It’s a rough-hewn outdoor hut — it looks a bit like an overly large Native American sweat lodge – perched on stilts directly over the steaming riverbed. The water comes out of the ground at about 104 degrees Fahrenheit, letting off clouds of sulphurous steam, which rise through the slatted wooden floor. It’s co-ed and bare-bones; you have to cross carefully to avoid burning your feet and choose a seat on a wooden bench depending on how much direct steam you can stand.

Next, a series of walkways and stairs leads you to the second “station,” the mud. On either side of the boardwalk, the mud blurps and burbles up from holes and fissures, marked by signs reading “peligroso” or, in English, “danger, hot mud.” Burlap sacks lie nearby; presumably this must be what they gather the mud in. I wonder how many hands have been scalded trying to complete this task.

On an open deck sit two huge clay vats of warm volcanic mud covered by loose wooden lids. You dip your hands in and start slathering; concerns about group hygiene have no place here. (I chose to assume that the sulphur and other natural chemicals would kill off any germs that tried to insinuate themselves.)

While waiting for the mud to dry, you can amuse yourself by trying to get close enough to photograph the monkeys. After washing it off in the outdoor showers, you move to the third area, the soaking pools. These are nothing like jacuzzis; they’re scalding stone pools cloudy with dissolved minerals. Cold water has been added to make them three different temperatures so that you can move from hot to cold to hot again, Japanese-style.

My fellow bathers include a young couple honeymooning from Atlanta, two elderly gentlemen from India, a middle-aged couple from Germany, and several fashionable young Ticos, as Costa Ricans call themselves. By the end of the day, having snapped numerous shots of each other slimy with muck, we’re all on a first-name basis and offering to share photos on Facebook.

Borinquen is a good choice for anyone who’s open to an authentically down-and-dirty volcanic hot springs experience. But this is only half the fun; there are also large luxurious non-mineral pools, a restaurant and bar, stables for horseback riding, and all the other amenties we’ve come to expect from a top-flight Costa Rican eco-resort.

The onsite spa has the woodsy elegance of a classic National Park lodge, frosted with a layer of zen. And a deep tissue massage after your muscles have been softened from hours of soaking in the mineral-rich springs? Unforgettable.

All photos by yours truly except the one of the mud-covered couple, who are clearly models.