At 370ft wide and 121ft deep, Grand Prismatic Spring is the park’s largest and deepest hot spring. It’s also probably the most beautiful single thermal feature in the park. Boardwalks lead around the multicolored mist of the gorgeous pool and its spectacularly colored rainbow rings of algae.
For the most dramatic photos of Grand Prismatic Spring, drive south to Fairy Falls trailhead, walk for 1 mile and then take a faint path up the side of the fire-burned ridge (itself a lava deposit from the west rim of the caldera). From above, the spring looks like a giant blue eye, weeping exquisite multicolored tears.
The spring drains into Excelsior Pool , a huge former geyser that blew itself out of existence in the 1880s with massive 300ft explosions of water. The last eruptions here were in 1985, when the pool erupted almost continuously for 46 hours, before lulling itself back into a deep sleep. The pool continually discharges an amazing 4000 gallons of boiling water a minute into the Firehole River – according to T Scott Bryan’s The Geysers of Yellowstone , that’s enough to fill 300,000 automobile gas tanks every day. More water is expelled here in a single day than Old Faithful releases in two months. You can admire the colorful yellow and orange runoff as you approach the basin over the bridge.
The features are linked by a 0.5-mile accessible boardwalk; budget about 30 minutes here.
