Just north of Mammoth, a roadside marker near the Gardner River marks the 45th parallel, halfway between the Equator and the North Pole. A short distance away is the Boiling River area, where hot runoff from the Mammoth terraces meets the cold river water.
This has long been one of Yellowstone's more unusual roadside stops, because the geology is right at your feet rather than behind a distant boardwalk. Access can change with floods, seasons, and park rules, so treat it as a place to check locally before building a day around it.