Geologists working in Yellowstone National Park discovered a new pool of strange light blue water this spring. Using satellite imagery, they have now been able to determine exactly how and when this new feature was born.

Yellowstone National Park closed the Madison River, the Firehole River and the Gibbon River downstream of Norris Campground, as well as those rivers’ tributaries, on July 12. Fishing draws about 50,000 visitors to Yellowstone each year, according to the park’s website.