Mudpots. Sounds like something a 3-year old kid makes in his or her backyard on a rainy day. Come to think of it, I’m sure that there are lots of small children who wander by these things in Yellowstone (hopefully firmly attached to one of their parents) who would love to stick their hands into this goo and play. They look harmless enough and they sound like some kind of porridge cooking on the stove. It’s really a very peaceful noise.
Peaceful sounding, maybe. Harmless, definitely not. The “mud” is actually liquefied rock and clay, dissolved by super-heated acidic steam. Putting your pinkies in there would probably result in some very serious injuries.
Photographically, these things are a bit troublesome. Trying to get an image that shows the bubbles bursting is kind of like trying to get a good image of lightning. You’re always a split second behind. So the best way to do it is to set the camera for multiple exposures – I set mine to take 4 or 5 on a single push of the shutter button. If you do that often enough, you’ll probably get a couple of “keepers” out of it. And make sure your shutter speed is fast enough (about 1/500 sec.). It’s best if you can freeze these little explosions of mud.