In the stock market, all is not quite as it seems.

A slowdown in inflation has boosted investor confidence in the economy this year and, combined with an intense fervor for artificial intelligence, provided the backdrop to a rally that has beaten all expectations.

The S&P 500 climbed about 15 percent in the first half of 2024, rising into record territory.

The gains have been remarkably steady, with the index only once rising or falling more than 2 percent in a single day. (It rose.) A widely tracked measure of bets on more volatility to come is close to its lowest-ever level.

But a look beneath the surface reveals much greater turbulence. Nvidia, for example, whose rising stock price helped it become the most valuable public company in America last week, is up more than 150 percent this year. The price has also repeatedly had deep plunges in the last six months, shaving billions of dollars of market value each time.

More than 200 companies, or roughly 40 percent of the stocks in the index, are at least 10 percent below their highest level of this year. Almost 300 companies, or roughly 60 percent of the index, are more than 10 percent above their low for the year. And each group includes 65 companies that have actually swung both ways.