I take it you mean low rumbles and such, and not explosions?
Well, the secret to really kick-ass bass-effects must always begin with exceptionally good monitoring. See, bass is pretty much imho the hardest thing in the entire realm of sound to get great, especially the lower you go below 100Hz. For consumers, that mostly goes pretty unnoticed because pretty much everything they ever listen to, that actually has that kind of bass, is already heavily contained and shaped. For us making said effects though it is not, and for those buying already shaped effects they still must balance it, which can be very difficult if the mix-room isn't well padded in the lows. And add to that that the human hearing reacts very differently to bass that higher frequencies, only a few decibels might very well be the difference between inaudible and unbearable in the sub-bass (sub-bass is freqs below approx 70Hz, the subwoofer is an LFE).
Now to the funny stuff: Don't let that let you down though. With a good and honest sub-woofer made for studio-work and nice acoustics this is literally a blast! ;-) How to reach these sound differ a lot between designers, though I'd say most of us work first and foremost with pitchshifting and bass-management like the Waves Air and such. Ta mention a few of the soundsources I often use to make dedicated bass-effects can be mentioned djembe-drums, dharbukas, large sheets of both a special plastic and different kinds of metal, a big auto-harp, and my own body. As it's often a pure matter of mic-positioning, you may find lotta really impressive sounds if you only play around with the mic in different ways for different sound sources.
A common misconception is that to make something fatter you need to add more bass, but the truth is that that often just makes things muddy. Though it might sound like a contradiction, one often gets the best bassy sounds by adding more of the higher frequensies, the human ear is extremely sensitive to details and good details in higher ranges makes the sound much more prominent.
Often though the best sounds are the ones created. Your best friend here is the pitchshifter. The best you can do is just experiment by yourself, but to give you a hint of how unlikely sources can sometimes become a human rear part full of Dr Martens (ie= kick ass like there's no tomorrow), a few years ago I messed around i the corridor outside my studio with something that's called "tricky-smällare" in Swedish. Dunnu what they're called in English, but they're a kind of small round crackers of thin paper that detonates on impact with a not exactly ear-threatening little "poff". I didn't really know what to do with it, but after a fun time of playing with my recordings through different pitchshifters and filters I actually got an amazingly convincing (and heavy) hearth-beat!