The Grand Prismatic Spring is located in the Midway Geyser Basin and aside from being the largest in the United States, it's also the third largest in the world. It is around 110 m in diameter and 50 m deep.
Because of its temperature (around 70 °C), the boiling water from underground is sterile, which makes the centre of the lake so clear and blue. The spring is framed by red, orange, yellow and green rings which are getting deeper or fade depending on the season. This unusual effect comes from the mats of colourful pigmented bacteria and microbes that live in the warm mineral-rich waters around the spring.
Although the Grand Prismatic Spring looks absolutely stunning, you shouldn't be tempted by its beauty and try to walk off the designated boardwalk. This boardwalk is made not only to protect visitors from the boiling liquid but also to keep the natural environment of the spring from damage. So while visiting this natural wonder make sure you're being careful and respectful to nature.
The best time of the year to visit the Grand Prismatic Spring is summer. The colours are at their brightest when the mats turn red and orange, and blue water contrasts with them. Moreover, summer months are generally better for getting around the park, and in terms of weather. Visitors also share some tips regarding the timing of the day, suggesting to be in the area at around noon for brighter colours and thus better pictures.