Geysers smell like sulphur, known in the olden days as brimstone. Since Yellowstone has the largest concentration of geysers on the planet, there are a lot of areas that smell like, well, really bad farts. But it's nature, right? So it's wonderful.
Besides the sulfur aroma, the northwest corner of Yellowstone currently smells like smoke, because of a fire. But that's sort of a good thing, like the way lighting a match helps counter the odors of flatulence. At least that's what I told myself.