Upper Geyser Basin has the largest concentration of geysers in the world. It is located about 28mi (45km) from the west entrance of Yellowstone. This is also where you'll find the popular, Old Faithful Geyser. It has regular eruptions that goes up to 185 feet (56 m). A schedule of the eruptions is located inside the Visitor Center or you can patiently wait in front of it, where tons of seating is available.
The Visitor Center is the only area that I found that had reliable WiFi.
Maps and a clear pathway is provided, which makes it easy to explore the two square miles area. Here you'll find several thermal features including: spouting geysers, hot springs and steaming fumaroles.
This is where I also shot at night and got a chance to see the northern lights. It was amazing because it never (rarely) occurs in this part of the world and it was so unexpected. My only intention was to see the milky way and with that, I got to experience this.
A neighbor of the Castle Geyser, this is the largest and hottest spring in the Upper Geyser Basin with a temperature of 199°F (93°C) and it boils continuously. The deep blue color and clarity of the water made it i the most beautiful hot spring I found in this area.