In many places around the world with the right conditions, there is hot water bubbling up from underground, coming to the surface as natural hot springs. While some of them are due to apparent volcanic activity, such as the impressive geysers in New Zealand I talked about here, they may also happen in regions with no volcanoes nearby, as is the case with the hot springs in Hungary. Usually, however, when naturally occurring hot waters are found, it's only a matter of time before it's turned into a spa. The best example for this are the Grutas de Tolantongo in Mexico.
Seeking Out Secret Hot-Spots
As much as I can enjoy tiled floors and stacks of fresh towels, my favorite kind of hot springs are the real natural, if not to say primitive, ones. The kinds where you have to hike for a few hours, and follow an obscurely drawn treasure map, only to discover for yourself: a natural pool, built of strategically placed rocks that had been lying around there in the first place, where the water can accumulate, inviting you to take a dip. Otherwise there are no facilities of any type - you are out in the middle of the wilderness, after all.
Being a fan of hot springs, I have taken the time to travel quite a bit to these kinds of natural hot tubs, and the Western United States are literally dotted with them. (Of course the dots are tiny on a map of immense size, but still you can say there are plenty of them.) Some of them are closer to roads than others, some may even have a river or a cold stream running nearby to cool off. The natural beauty surrounding them is always astonishing. Unfortunately I don't have any pics to share, and no, I'm not going to give you exact locations either. But ask around, and local fans of hot springs will be happy to point you in the right direction.
Las Grutas de Tolantongo
Instead, I want to tell you about a similar hot springs in Mexico, which started out as something like this, and by now has developed into a veritable spa. When I first came to Mexico, the place was not even mentioned in the guidebooks all the backpackers were using. Instead, I was told about it by some Mexican hot springs fans. The instructions of how to find it had the usual obscurity: "From the town of Ixmiquilpan in the state of Hidalgo you have to take the road leaving North, until the hamlet of Molanguito. There you take the side road down into the canyon. By the time you reach the river you will find the hot springs."
So with a group of adventurous friends we set out, and using mostly the local colectivos we descended down into the canyon. It was impressive how in the middle of the bone dry desert the bottom of the canyon looked lush, green, and inviting. Once we were there, we could see the different features of the place: cold waters cascading down in a waterfall over a cave, from where the hot waters were running out. Eventually the two streams flowed both into a river, which was just as cold as the cascades.
Back then the facilities were minimal, though there were stairs cut into the hill to facilitate the access to the cave. Inside the cave was the real hot pool, and we could even go quite deep into it, so much so that eventually we were in complete darkness, while still enjoying the pleasant hot water. On the way in - as well as on the way out - the natural cold shower was inevitable. This is so perfectly in the spirit of the art of bathing, that it looks as if nature had fully intended us to enjoy this natural hydrotherapy the right way.
When a Spa Springs Up at the Hot Spring
My experience in Tolantongo was so nice that the following year I visited again with the woman who later became my wife. By then there was a lot more touristic activity. Not just had the handful of visitors increased to a small crowd, but there was a camp site on the side of the hill, and they had started building a couple of pools below the waterfall. Not that I can blame them, after all it seemed like the word had gotten out about the place, and they locals wanted to accommodate the visitors (and make a buck along the way).
However, doing a web search on Tolantongo now, more than 15 years later, I can't even recognize the place!!! There are numerous pools, on top of each other, presumably with varying temperatures, and the map indicates numerous hotels along the river. Due to a funny coincidence, another Hivean 
One thing is certain, Tolantongo will never be the same again, making me glad I visited back when I did. Still, I can't say I the place looks bad. Obviously, the other developed spas I have written about in my series are all worth a visit, in spite of the facilities built around them. I just wish I had taken more photos back then.