Mystery solved: Trails of light illuminate skies 

Social media lit up on Monday as people in Catalonia and beyond shared images of strange, zig-zagging trails of light in the sky.  

"What could this trail be that was seen in the sky this evening from Tarragona?" one Twitter user asked. 

The truth was out there, as Dr. Josep M Trigo, an astrophysicist & planetary scientist at the Institute of Space Sciences (CSIC-IEEC) in Barcelona, explained.  

The light "was produced by the trail left by a test [on Monday] of a new French cruise missile," Trigo posted on Twitter. 

Residents in Catalonia were able to see it "for hours" due to "favorable geometry of solar light" he added.  

Indeed, French defense officials had issued a warning about the possible launch of its new V-MAX hypersonic glide vehicle in the Bay of Biscay. 

Mystery solved.