The Hubble Space Telescope has spied a dark vortex on the planet Neptune, a spot in the ice giant’s atmosphere the size of the continental United States.

Dark spots on the planet are high-pressure systems which usually are found hand-in-hand with bright clouds, NASA says. Indeed, astronomers saw such bright clouds in 2015, and the Hubble itself saw a dark spot near them that same year. Then, in May 2016, Hubble saw the dark vortex again, confirming its previous observation.

"Dark vortices coast through the atmosphere like huge, lens-shaped gaseous mountains,"  Mike Wong, a University of California at Berkeley research astronomer, said in a statement. "And the companion clouds are similar to so-called orographic clouds that appear as pancake-shaped features lingering over mountains on Earth."

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Neptune is so far away from the sun (about 2.8 billion miles) that it takes an incredibly long time for one year on it to pass— the blue, ringed planet takes nearly 165 Earth years to make one trip around the sun.

Dark spots have been seen before: in 1989, by the Voyager spacecraft, and in 1994, by Hubble. This latest one, now observed two years in a row, is estimated to be about 3000 miles across.