The water of boiling springs at Yellowstone was not always florescent, it was blue. The change in color over the decades can be attributed to pollution and trash thrown by tourists. When Yellowstone was made a national park in 1872, the pond boasted of deep blue color.
These pretty thermal springs have been around for a long time and the wonderful view attracts millions of tourists from all over the world. It has been observed that the colors of the springs have been changing. And scientists have confirmed that it is true.