A darkly colored storm on Neptune that’s the diameter of the Atlantic Ocean is shrinking. 

The Hubble Space Telescope first spotted the dying storm in 2015 and has kept an eye on it over the past few years. The most recent photos from the Hubble show that the storm is shrinking. 

This isn’t totally unexpected. While storms like Jupiter’s Great Red Spot have been raging for centuries, Neptune’s storms tend to be more short-lived, NASA said. 

Oh, also, the dark storm may smell like rotten eggs. 

According to NASA, the storm is probably made of hydrogen sulfide, meaning that, yes, it probably has that distinct, gassy smell. 

The storm’s “particles themselves are still highly reflective; they are just slightly darker than the particles in the surrounding atmosphere,” Joshua Tollefson from the University of California at Berkeley said in a statement.

Although scientists understand that the storm is shrinking, Neptune’s tempests still hold some mysteries. 

First of all, researchers aren’t exactly sure how these storms form in the first place — though they do have some guesses.

“We have no evidence of how these vortices are formed or how fast they rotate,” Agustín Sánchez-Lavega from the University of the Basque Country, said in the statement. “It is most likely that they arise from an instability in the sheared eastward and westward winds.”

Also, the storm isn’t dying in the way the scientists expected. 

Earlier simulations showing the deaths of these kinds of storms suggested that “anticyclones under Neptune’s wind shear would probably drift toward the equator,” Michael Wong of the University of California at Berkeley, said in the statement. 

“We thought that once the vortex got too close to the equator, it would break up and perhaps create a spectacular outburst of cloud activity.”

That wasn’t the case. Instead, the storm seems like it’s just fading into nothing as it moves toward Neptune’s south pole and not the planet’s equator as expected, NASA said. 

Storms like this one on Neptune were first spotted by the Voyager 2 spacecraft during its trip through the solar system. Today, the Hubble is the only spacecraft that can keep a good eye on the storms and their motions.