A couple more from Yellowstone...

Nikon D90, 18-105mm

One of Yellowstone's mud-pots - basically a volcanic gas vent that's filled with mud (instead of water as in geysers). So, it bubbles and spits and burps, instead of hissing or spraying.

Nikon D90, 18-105mm

A hot spring, near Old Faithful. No bubbling or hissing. Just some pretty blue water, simmering away in its deep mineral-lined pool. It smelled like a smoke bomb.