The road network in Yellowstone National Park is laid out in a massive figure-8 with a north and south loop. The west side of the south loop connects Grant Village with Maddison Campground.
From Grant Village at the south end of the south loop, the road crosses the Continental Divide three times before traversing Upper, Midway and Lower Geyser Basin. In the process, travelers are treated to two incredible geological formations: Old Faithful and Grand Prismatic Spring.
Old Faithful is situated in the Upper Geyser Basin and has a well-marked exit off the main road. The complex is sprawling, complete with lodging, post office, gas station, grocery store, visitor education center, restaurants and, of course, Old Faithful.
You might be surprised to learn that most of Yellowstone National Park has no cellular service. This makes the Old Faithful area an important exception if you really need to get a call out. I didn't have LTE but I had full bars and was easily able to make a few phone calls..
A few miles north of Old Faithful is Midway Geyser Basin and Grand Prismatic String. It's almost impossible to miss because it's very popular and there is a lot of traffic going in and out of the parking area. On this day, there were huge clouds of steam waifing over the road from the thermal waters percolating up from the spring.
I parked in the parking lot and took the boardwalk up to and around the loop. The boardwalk takes visitors by Grans Prismatic Spring and Excelsior Geyser, a churning pool of thermal blue water constantly bellowing steam. It's a small hike, but totally worth the effort. Grand Prismatic Spring area is an amazing and etherial collection of pools, colors, landscapes and smells.