My boyfriend and I went on a half day excursion to the mud volcano outside of Cartagena with a tour arranged through our hotel (Don Pedro de Heredia - recommended). Here is the quick and dirty: this is not a relaxing, medicinal, or even sanitary experience. But it is fun, and if you have the stomach for it and a strong immune system, maybe worth doing. In retrospect, I personally would have skipped it if I knew what I was in for, but at the same time it made for some very funny experieneces and pictures.
We spent most of our twenty minutes in the volcano laughing - my boyfriend got in first and despite efforts to stay upright, quickly toppled on his back and couldn't get vertical again because the mud is so buoyant. He then received an "optional" massage ($3 - 2,000 pesos - why not) that was impossibe to refuse because you really can't move yourself around in the mud and you're at the attendants' mercy. They rubbed his legs and arms up and down a few times - not sure the massage would win any awards but part of the experience.
I got in next and the same thing happened to me. My boyfriend and I then hung out for a while and tried to dunk each other (unsuccessfully) and laughed with all the other tourists soaking in the thick, lukewarm mud. All the while, a local who had offered to hold our camera and take photos snapped some shots (also $2000 pesos - or $3 - I believe) and had our shoes waiting for us at the bottom of the volcano when we decided to climb out of the mud and down the precarious, slippery staircase to rinse off.
What was gross about this (part one): The mud is that lukewarm temperature that's probably a breeding ground for all kinds of things I'd rather not think about, and we're not exactly talking about filtered, "spa" mud here - this is essentially a big hole in the ground with mud that thousands of people have sat in and done Lord knows what in. The mud is sort of granular and has little "mystery bits" of who knows what. So just know what you're getting into. You're going to sit in a moist pit in a small, rural outpost in a developing country. Not to say that you shouldn't, but if you're a clean freak / germophobe, this isn't for you.
What happens next: After climbing down the volcano, we were then shuttled to rinse off in a small, brown, stagnant freshwater pond. Several ladies quickly surrounded us and started throwing buckets of water over our heads for our "optional" ($3) rinsing service - my bikini top was removed (30% my choice, 70% insistence of the attendants) and I held firm on my bottoms. They scrubbed us down and helped us get the mud off, and also ensured that plenty of the brown water was thrown directly into my eyes, ears, and nose.
What was gross about this (part II): See above description. Also, you will not feel entirely clean until you get home and spend a good 45 minutes in the shower removing mud from every last hiding spot.
After our "rinse" we went with the group to a lunch on a nearby beach (food actually quite good - soup, a whole fried fish, soda, and coconut rice) and then home after a short stint in the sun. The beach is nothing special (and actually, our guide described the water as "sucio" in her Spanish version of the instructions but didn't include "dirty" in her English version) so it's not a great place to go swimming. But it would have been a very quick trip without the lunch so I didn't mind the detour. The only downside was that it delayed our chance to get in the hotel shower and scrub off the remains of our spa experience.