Nikolai Kondratiev was a Russian economist with the misfortune to have Stalin as a boss; he was caught up in the purges and eventually executed in 1938. I was reading some article about "1o people who predicted the crash" or something to that effect and he was mentioned, so I quickly looked up the name (the Internet -- particularly Google -- makes this very easy, but also shallow -- see "Is Google Making Us Stupid?" by Nick Carr).
It turns out that before Kondratiev was imprisoned in the Gulag he developed a theory that the activity of capitalist economies can be charted in long waves, or supercycles, and that these cycles are far longer and deeper than what is normally characterized as the 'business cycle': According to the theory, the timing of these deep boom and bust cycles is 50 years or more...
A chart illustrating two hundred years of economic activity.... hmm, what's next? The graphic is from a site entitled Kondratieff Winter that contains additional information about the application of this theory to the present situation. The site was launched in 2007, when, as the site owner notes, the stock market was at 14,000. As usual, wikipedia also has an entry of this subject.
I don't know, it's very disconcerting, maybe I am not brave enough but I live in fear. Not paralyzing fear, but a giddy prickly sensation when I leave for work in the morning and a slightly queasy feeling when I walk into my condo unit at night, and I lie awake in the early hours of the morning and wonder what comes next. And I am not even that heavily exposed (I've never been a fan of borrowing money) by the standards of some of my neighbors...but was it enough?