The Grand Canyon has had two temperature inversions over the last few days, causing the entire canyon to fill with a sea of fog. “It’s a once in a lifetime, outstanding, crazy, amazing, mind blowing inversion,” explains the official Grand Canyon National Part Facebook page.

Under normal circumstances, cooler air is lighter and rests atop warmer air, so that the temperature falls as elevation rises, the National Weather Service explains. A temperature inversion is when warm air acts as a lid and traps cold air near the surface. This most often occurs overnight when surface temperatures are at their lowest. Calm winds, long nights and clear skies also encourage inversions.

Such an occurrence typically happens at the Grand Canyon once or twice a year. These recent inversions are particularly special, however, because the clouds filled the entire canyon rather than only parts of it and because the events occurred on clear days. Park ranger Erin Whittaker told MailOnline that such a complete inversion happens only once a decade. “Rangers wait for years to see it,” she wrote on the Facebook page. “Word spread like wildfire and most ran to the rim to photograph it. What a fantastic treat for all!”

Some visitors who sought the traditional view of the canyon were actually disappointed by the phenomenon, believing it to be typical fog and nothing out of the ordinary. But visitors didn’t have to wait long for a clear view. Whittaker explained on Facebook, “By the end of the day the sun was able to burn it all away.”