This week marked a major financial milestone: The S&P 500 set a record for the longest bull market in history. US stocks have risen steadily since March 2009, without ever slipping into bear territory (defined as a fall of 20% or more). Over that period, the index has gained some 320%.

Instead of celebrating, some investors are getting nervous. The record feels like an ominous sign that the good times will soon come to an end. Because if there’s one prediction we can make with certainty, it’s that the market will fall eventually—the only question is when.

It’s true that past bull markets did not last this long. Most previous runs were shorter and stronger, whereas this bull market has featured long periods of low volatility and modest returns. The average annual return since the bull market began is 16.5%, ranking 10th out of 13 bull markets since the 1930s.

While it’s always dangerous to say “this time is different,” maybe markets do behave differently in the 21st century. More globalization leads to more diversification and less risk, which could translate into a more stable and steadily rising stock market. More finely tuned monetary policies could also be a factor. We may be entering an era of weaker but longer-lasting bull markets. In that case, stock prices could continue rising for even longer.

Or, the past nine years have been a fluke and a major correction is around the corner. The factors that reduce volatility may generate longer periods of slow, steady growth most of the time, but they could also increase the odds of tail risk. That is, there could be fewer crashes than before, but the ones that do come are far more severe than usual.

Ultimately, the market is impossible to predict; don’t believe anyone who says otherwise. The best you can do is hold a diversified portfolio of low-cost index funds and steel yourself for the reality that there will be good times and bad times but, over time, the stock market mostly charges ahead.