Sapphire Pool

Sapphire Pool, located in the heart of Biscuit Basin in Yellowstone National Park, is one of the park’s most mesmerizing hydrothermal features. True to its name, the pool glows with a brilliant sapphire-blue color that reflects the purity and depth of its hot waters. Surrounded by white sinter formations and vibrant thermophilic life, Sapphire Pool presents a breathtaking example of nature’s artistry and Yellowstone’s powerful geothermal activity. Though it appears serene today, the pool’s tranquil surface hides a dramatic past of eruptions, earthquakes, and transformation.

Sapphire Pool

Sapphire Pool

Quick Reference: Sapphire Pool

Feature	Details
Location	Biscuit Basin, Upper Geyser Basin, Yellowstone National Park, Wyoming
Type	Hot Spring (formerly a geysering pool)
Color	Deep sapphire blue with yellow and orange thermophilic borders
Average Temperature	159.8°F (71°C)
Average pH	7.9
Average Conductivity	2239 µS/cm
Depth	Approximately 35–40 feet (estimated)
Elevation	About 7,300 feet (2,225 meters)
Activity History	Actively geysered before 1959; major eruptions followed the 1959 Hebgen Lake Earthquake; last known eruption in 1991; currently inactive
Formation	Sinter-lined pool formed by silica deposition from geothermal waters
Coloration Source	Blue color from depth and clarity; yellow and orange hues from thermophiles in runoff channels
Accessibility	Easily accessible via the Biscuit Basin boardwalk, north of Old Faithful
Visitor Tips	Best viewed on calm mornings for clear reflections; stay on the boardwalk to avoid fragile sinter crust
Nearby Features	Mustard Spring, Jewel Geyser, Avoca Spring, and the Firehole River
Interesting Fact	Sapphire Pool’s violent post-1959 eruptions destroyed the biscuit-like geyserite formations that once gave Biscuit Basin its name.

A large, clear blue pool, Sapphire Pool features a solid sinter rim level with the ground surface, giving it a perfectly sculpted, natural basin. Along the edges, shallow sinter shelves host colorful communities of yellow and orange thermophiles, whose pigments create a striking contrast against the deep blue water. The pool’s clarity and symmetry make it one of the most photogenic spots in Biscuit Basin, especially in the early morning when steam rises gracefully from the surface, glowing in the sunlight.

Silex-rich water also contributes to Sapphire Pool’s beauty. As superheated water circulates through the volcanic rocks below, it dissolves silica and other minerals. When the water reaches the surface, it cools and releases this silica, which hardens into the sinter that lines the pool and forms its terraces. Over time, this mineral buildup creates the smooth, reflective surfaces that make the pool look like a mirror.

Before the great earthquake of 1959, Sapphire Pool was primarily a placid, blue spring, its edges surrounded by unique biscuit-like geyserite formations that gave Biscuit Basin its name. These formations, built from deposits of silica and minerals carried by the hot water, were delicate and beautifully sculpted. However, the 1959 Hebgen Lake earthquake triggered intense geothermal disturbances across the region, and Sapphire Pool responded with several powerful eruptions. These eruptions destroyed the original biscuit-like formations around its edge, permanently altering the pool’s appearance. Following this period of violent activity, the pool filled with muddied water and took on a completely different appearance.

It wasn’t until 1971 that Sapphire Pool’s water cleared again, restoring its brilliant blue hue. The pool’s eruptions ceased, signaling another major shift in its underground connections. Since then, Sapphire Pool has remained mostly quiet. The last recorded eruption occurred in 1991, and the minor geysering that was once common before 1959 has not returned. Geologists believe the earthquake permanently altered the plumbing system beneath the pool, cutting off the conditions necessary for eruptions while allowing it to remain a steady, steaming spring.

Sapphire Pool has an average temperature of 159.8°F (71°C), an average pH of 7.9, and an average conductivity of 2239 µS/cm, indicating the mineral richness of its waters. The pool’s delicate balance of chemistry, heat, and biology makes it a key site for studying Yellowstone’s geothermal ecosystem.

Visitors can easily reach Sapphire Pool via the Biscuit Basin boardwalk, located just a short drive north of Old Faithful. The path provides safe, up-close views of the pool while preserving the fragile ground. On calm days, steam gently drifts upward, and reflections of the sky shimmer across the blue water, creating one of Yellowstone’s most tranquil and captivating sights.

Though it no longer erupts, Sapphire Pool continues to capture the imagination of all who visit. It stands as a living testament to Yellowstone’s ever-changing nature—where beauty, power, and transformation coexist beneath the surface of the Earth’s most dynamic geothermal landscape.