Remember that bizarre "magic island" on Titan's northern seas? The one that suddenly emerged and then inexplicably disappeared? Well, it's baaa-aaack — and it's changed.

So this thing — whatever it is — is clearly evolving. NASA's Cassini spacecraft has been monitoring the progress of the mysterious feature, which resides in a large hydrocarbon sea on Saturn's moon Titan. It appears to cover an area about 100 square miles (260 square km) in Ligeia Mare, one of Titan's largest seas. To date, it has been observed twice by Cassini's radar equipment, but its appearance has changed between the two observations.

The feature was first spotted during a Cassini flyby in July 2013. Previous observations showed no signs of it at that location in Ligeia Mare. Strangely, it disappeared during the next several months, leading some team members to suggest it might be a transient phenomenon; it could be a seasonal feature. And in fact, the moon is transitioning from spring into summer (which it will do by 2017) leading to increasing rates of solar activity on the region.

During Cassini's flyby on August 21, 2014, the feature was once again visible — and its appearance has clearly changed during the 11 months since it was last seen.

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So what is it? It could be a number of things, including surface waves, rising bubbles, floating solids, solids suspended just below the surface, or perhaps something more exotic.

"Science loves a mystery, and with this enigmatic feature, we have a thrilling example of ongoing change on Titan," noted Cassini team member Stephen Wall in a NASA statement. "We're hopeful that we'll be able to continue watching the changes unfold and gain insights about what's going on in that alien sea."

Image Credit: NASA/JPL-Caltech/ASI/Cornell