Grand Prismatic Spring, Thermal spring in Yellowstone National Park, US.

Grand Prismatic Spring is a hot spring in Yellowstone's Midway Geyser Basin, about 370 feet (110 m) across and 160 feet (50 m) deep. Its center is deep blue, while orange, rust-brown, yellow and green rings of microbial mats surround the water, edged by a pale yellow mineral rim.

Geologists from the Hayden Expedition named the spring in 1871 after documenting its rainbow-colored formations. Yellowstone became the world's first national park a year later, securing protection for this thermal feature.

The vivid color rings form from tiny organisms that thrive at different temperatures, giving the water its bright hues. Visitors see this natural palette best on clear days when sunlight hits the surface directly.

Wooden boardwalks lead close to the spring and offer safe viewing points, while an elevated trail to Fairy Falls Trail provides a view from above. The colors appear strongest in bright sunlight, usually between late morning and early afternoon.

The water flows continuously and discharges roughly 2,100 liters per minute, causing the color patterns to shift slightly throughout the day. On cool mornings, steam rises and temporarily hides the colored edges while the center remains visible.