Mammoth Hot Springs has many fascinating geologic features created by the earth's hot magma being so near the surface and affecting the mineral content of the surface materials. In most places the terrain is other-earthly, unfit for human activity (except tourism) and smells of sulphur. The smell of sulphur is the smell of rotten eggs.
Mammoth Hot Springs is also the site of Fort Yellowstone and a hotel. It is a packed focal point. I went to the hotel lobby to check email and was told wifi is only available in the restaurant lobby after 5 p.m. Okay. I walked over to the restaurant, found the lobby was a bar and pulled out my iPad. The wifi was not free. Nothing there is free (except parking if you can find it.) So I skipped emails. The sightseeing was fun.