NASA has detected a strange dark vortex on Neptune's surface. Images sent from the Hubble Space Telescope have revealed that a dark spot suddenly opened on the planet's surface and it's perplexing astronomers.

According to NASA, this is the first observation of a dark vortex in Neptune in the 21st century. Past observations have occurred in 1989 via the Voyager 2 and in 1994 via the Hubble Telescope. The most famous dark spot discovery in Neptune was in 1989. Tagged as the Great Dark Spot, it was located in Neptune's southern atmosphere and is approximately as large as the Earth, The Verge reports.

Dark vortices are formed in Neptune when clouds of air and gas swirl and freeze up. This creates a solid single mass that moves on the planet's atmosphere.

Mike Wong, leader of the study from the University of California-Berkley, described these dark vortices as "huge, lens-shaped gaseous mountains” that move through the atmosphere. Gizmodo says that the dark vortices appearances, such as their size, shape and lifespan, all differ. However, what's always constant is the stream of pancake-shaped bright clouds accompanying them called orographic clouds.

The Outer Planet Atmospheres Legacy (OPAL) captured the said dark spot on Neptune in September 2015. To better observe the phenomenon, the team created a higher-quality map of the dark vortex and its surrounding using the new images from Hubble. Wong and his team announced the discovery on May 17, 2016 in a Central Bureau for Astronomical Telegrams (CBAT) electronic telegram.

There is still limited data about dark vortices. Through this third sighting, scientists in NASA hope to learn more about the origin of dark vortices, their behavior as well as how they interact with Neptune's surroundings through continued observation.

To know more interesting facts about Neptune, check out the video below.