Like many other places in Yellowstone, the Firehole River is a scenic spot that lives up to its dramatic name. As it meanders north for 21 miles to join with the Madison River, the Firehole acts as a drainage basin for many of the park’s geothermal features. In collecting all of the mineral rich water, which has been naturally heated by the earth, the Firehole River can run 15 degrees warmer than surrounding rivers and lakes—with the steam that’s often emitted from the surface being one of the reasons for its name. Despite its boiling appearance, however, water temperatures are still fairly moderate at around 75°F (23.8°C), which makes it a comfortable temperature for splashing at the Firehole River swimming hole.
In 1870, when members of the Washburn Expedition happened upon this Yellowstone geyser, they noted the shape and structure of the crater resembled the tower of a castle. Today, much of that stoic castle tower has gradually dissolved and eroded, although a 90‐foot (27-meter) column of boiling water still erupts with regular frequency. On average, the Castle Geyser has an eruption cycle in the vicinity of 10‐12 hours, and eruptions will last for 20 minutes before the water changes to steam. As opposed to nearby Old Faithful, however, predicting the timing of Castle Geyser can be a little bit tougher, with the tradeoff being the Castle’s roar when the water switches to steam. For the first 15 minutes of Castle Geyser’s steam phase, visitors may experience a thunderous roar like the sound of an oncoming train, as the geyser releases its pent up energy and belches a stream of thick white steam towards the open blue sky above.
Black Sand Basin is one of the shortest walks in Yellowstone National Park—but that doesn’t mean it’s short on sights or spectacular things to see. Rather, this volcanically heated, bubbling basin is a packed with geysers and pools, from the periwinkle colored Opalescent Pool, to the Spouter Geyser which consistently erupts for up to 10 hours at a time. At Emerald Pool, where the water temperature is slightly lower than neighboring Rainbow Pool, the water casts a deep green hue of molten emeralds, whereas Rainbow Pool is slightly warmer and tinged with yellow and orange. Occasionally, depending upon eruptions, the Cliff Geyser will come to life and send water 40 feet high, although unlike Old Faithful just one mile south, the interval is irregular enough to the point you should consider yourself lucky if you see it erupt. As for how it acquired its name, Black Sand Basin is named for obsidian that’s been crushed down into the earth.