The Grand Prismatic Spring is Yellowstone National Park’s most famous hot springs. It is the largest single hot spring in the park and the world’s third largest. Grand Prismatic Spring is located in the Midway Geyser Basin.
The hot spring can be seen from the boardwalk trail through the Midway Geyser Basin as well as the Grand Prismatic Spring Overlook Trail. It has a diameter of approximately 370 feet, which is wider than a football field. The spring has a depth of over 121 feet. This is the famous view from the overlook:
The colors of the spring are caused by bacteria and thermophiles (a bacterium that grows best at high temperatures). Each of the heat-loving algae prefer a particular temperature, causing the different colors as the water cools away from the center. They follow the spectrum of white light through a prism, from red to blue like a rainbow.
The water at the center of the pool is around 189 degrees Fahrenheit, which is too hot for most organisms. The water looks very clear and a beautiful deep blue as a result of the water scattering the blue wavelengths.
Midway Geyser Basin Trail
This 3/4 mile loop trail takes visitors over a bridge across the Firehole River to see the geysers and hot springs of the area up close. Due to the proximity of the boardwalk to the Grand Prismatic Spring as well as its size, it can be difficult to appreciate the beauty of the hot spring in the same way as the overlook. The steam caused by the difference in the temperature between the water and air can also obscure the colors. For this reason, the best viewing is during hot summer days when there will be the least fog/steam.
The Grand Prismatic Spring Overlook Trail starts at the Fairy Falls Trailhead and climbs 105 feet in elevation up the Midway Bluff over .6 miles to the overlook. Hikers can look down on the Grand Prismatic Spring and the Midway Geyser Basin. The overlook is where the amazing rainbow color photos of the spring are taken. A viewing platform was completed and opened in 2017.
The trailhead is one mile south of the Midway Geyser Basin, where a steel bridge crosses the Firehole River. It is about six miles north of Old Faithful. The trail is in a Bear Management Area so it is usually closed from early March until Memorial Day weekend.
It is another mile on the trail past the observation point to Fairy Falls, a 200 foot high waterfall considered one of the best in the park. The total roundtrip to Fairy Falls is 3.2 miles.
Do not leave the boardwalk around Grand Prismatic Spring, as the hot spring will quickly cause severe burns and even death. Additionally, do not fly a drone there (or anyone in the national parks). Not only is flying private drones banned in the parks, they may crash in the hot spring and cause severe damage to the ecological balance. Both activities can result in high fines.