Agreed. When I was young, I was a bit of a pyro and I loved coming up with homemade bombs, thermite, etc. Even before widespread use of the Internet, all you needed was to download a copy of the Anarchist's Cookbook from a BBS and you had plenty to get you started. I suspect that with a couple months of work and several thousand bucks, I could build an air cannon that would fairly consistently deliver 10-pound home-made explosives to explode on impact, enough of a distance away to carry out that attack..... assuming I didn't blow myself up in the process of figuring it out.
That's why what really puzzles me is how few highly deadly terrorists attacks there have been. Clearly there are a handful of people perfectly willing to kill lots of random strangers, and willing to risk certain or highly likely death to do so. Yet most of them pick stupid plans, thank goodness.
Take the Boston Marathon bombing as an example. Of all the Boston events you could have blown up at the time, that was one of the worst for the method they used, since the nature of the event strung people out along a thin line over the course of 26 miles. Even at the more densely-crowded finish line, there just wouldn't be that many people in a blast radius. Compare Boston's marathon finish line to a typical crowd on the Esplanade for the July 4th concert:
I guess we're lucky that most of those we're up against are idiots.