Antimatter Lightning! We're going to do our best, but nothing we could possibly write will be as awesome as the fact that's real and on Earth. In fact, nothing you ever read for the rest of your life will be as awesome as that, and it's only the way human memory degrades with time that'll let you ever get excited about anything ever again.
Like all the most discoveries, it was unexpected. The Fermi Gamma-Ray Space Telescope was launched to examine the universe for the stupendously powerful processes that produce gamma ray bursts, from black hole jets to the effects of dark matter itself, only to find blasts coming from behind it. From Earth.
That large storms can produce gamma rays isn't news, but the surprise - and that's "Surprise" with a capital "Antimatter Annihilating Tiny Bits Of The Atmosphere In The Most Energetic Reaction Known To Science" - was that some of the Terrestrial Gamma Flashes (TGFs) were of the exact energy level which could only be produced by positron annihilation: anti-electrons appearing, meeting and electron, and exploding. (And you'll never see a TGiFs without thinking of how awesome science is ever again).
How can this happen? No-one knows. Michael Briggs, research scientist at the University of Alabama, first announced the results at the 2009 Fermi Symposium and is currently preparing a paper on the subject. After that it'll be a race to see who can explain the events - if a slightly less explosive battle than you'd expect with antimatter and lightning bolts involved.