Lightning
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Lightning is one of the most difficult but rewarding phenomena to catch. It occurs in the briefest amount of time, and during a rainstorm, so you're lucky if you catch it and you have to be under cover to do so. A photographer back when I was in middle school told me that the secret to photographing Lightning was long exposures of summer storms. The long exposure (but not too long otherwise contaminating light would eliminate the lightning) would allow you a much better chance of catching the lightning, and summer storms usually are farther away from you but still with visible lightning.
So on a field trip around a few states during my first semester of graduate school, there was a summer storm in the distance one night while we were camping in New Mexico. I set up my Canon EOS Digital Rebel on my tripod and took about two dozen 2-minute exposures. I only caught one really good blast, but it was still worth it. Clicking on the image gives a bigger image, but it also shows the whole field (which was with an 18 mm lens).