Zooites at the Higgs Hunters project flagged this particular image as interesting, because, despite virtually no particle tracks appearing, loads of activity is visible in the muon detector (the outermost ring of the image).
Scientist Andy Haas explains exactly what’s going on here, and is it ever cool:
“This is a fun event. What has happened is that a very energetic cosmic ray (muon) has showered in the atmosphere or rock above ATLAS creating hundreds of lower-energy muons traveling in the same direction at the same time. You’ve found a particle from outer-space (probably a supernova!), whose energy is greater than we can create at the LHC!”
Andy also gave us a little bonus:
“I’ve attached a zoomed-out slice view, where I’ve also drawn the short muon “segments” found. (They usually don’t make full tracks since they don’t line up with the proton collision point.)”
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Daily Zooniverse 
Awesome!
“[…] found a particle from outer-space (probably a supernova!)” Hmm … if it were a sufficiently energetic particle from space (unlikely, seeing as how rare they are), even a supernova could not have created it. Rather, it would mostly likely have come from an AGN (active galactic nucleus), where a jet with enough energy in one second to power the LHC for a trillion years, a jet aimed straight at us, accelerated a particle to a billion times the energy of any proton in the LHC.
Anyone interested in finding such makes-the-LHC-seem-like-nothing natural particle accelerators? Head on over to Radio Galaxy Zoo; there are hundreds waiting for you to discover! 🙂