The 30th anniversary of Black Monday is this week. On Oct. 19, 1987, the Dow Jones Industrial Average plunged 508 points, or 22.6%, in a single session. A new book about the event, “A First-Class Catastrophe: The Road to Black Monday, the Worst Day in Wall Street History,” argues that we still haven’t learned the lessons from that massive meltdown, even through the crashes that followed it.

Certainly, Wall Street’s current buoyancy is a stark contrast to that dark day. The S&P 500 hit historic highs for six straight sessions earlier this month—the longest such streak in a generation. All told, the benchmark U.S. stock index has gained about 280% since the “Haines bottom,” the stock market low on March 9, 2009 during the global financial crisis.

Yet this does appear to be an unloved, or at least untrusted, bull market. According to longtime market strategist Richard Bernstein, the 2008 bear market and recession appears to have permanently damaged investors’ psyche. Today’s investors—including big pension funds, endowments and hedge funds—“remain more focused on limiting the downside risk of public equity holdings than on potential opportunities.”

Why worry, given all of Wall Street’s recent record-breaking? After all, the U.S. has low unemployment (4.2%), strong corporate profits and low market volatility.

But this bull run, already the second-longest ever, is overdue for a correction by historical standards. By many metrics, valuations are stretched. The market’s leadership has been relatively narrow, often dominated by a select few stocks like the FANGs (Facebook, Apple, Netflix, Google). And interest rates can only go up, which is generally bad news for the stock market.

What might spark a downturn? The proximate causes in 1987, 1997 and 2008 were events that few “experts” saw coming: automated computerized trading run amok, an “Asian contagion” and the startling collapse of Lehman Brothers. The next one could be a “Brexit”-style geopolitical earthquake, an economic “hard landing” in China, war with North Korea or a leadership vacuum at the Federal Reserve. But if history is any guide, it will be something few people are fretting over today—another “black swan” event that is obvious only in hindsight.