Even a strong bull market can't rise forever, and today saw a modest decline for stocks overall. Internationally, Asian markets were mixed, but European stocks fell much more sharply than in the U.S., with mining companies especially concerned about a Chinese slowdown. U.S. stocks, meanwhile, recovered some of their early losses, and the Dow Jones Industrials (INDEX: ^DJI) finished down just 69 points, to 13,170. Both oil and gold prices fell as well.

Some stocks had even steeper drops today. Let's take a closer look at three of them.

Caterpillar (NYSE: CAT), down 2.6%
At first glance, today should have been a good day for Caterpillar. The company announced that sales rose 20% in its fast-growing Asia/Pacific region and a whopping 39% in its North America region in February.

But the problem is that those growth rates are slowing quickly. As recently as December, U.S. sales were rising at a 51% clip, and Asia/Pacific growth was at 31%. Investors are clearly taking the news as evidence that a Chinese slowdown is real and could hurt the company going forward.

Alcoa (NYSE: AA), down 1.6%
Aluminum maker Alcoa is economically sensitive enough that you can almost count on it to drop whenever bad news about China hits the headlines. But as the first quarter draws to a close, many people are starting to look to next month's earnings season.

For Alcoa, the future looks a lot less shiny than it did at the beginning of the year. Consensus estimates for this quarter have dropped from a projected $0.14 per share profit as of three months ago to a loss of $0.04 per share today. Full-year 2012 earnings estimates have been cut almost in half. Unless Alcoa can turn things around quickly, its slump could last a whole lot longer than shareholders want.

United Technologies (NYSE: UTX), down 1.6%
Even though United Technologies is a conglomerate, investors seem to push it around as though it were simply a defense stock. Today's drop is apparently due to reports of cost overruns on Lockheed Martin's (NYSE: LMT) F-35 fighter jet, for which United Technologies' Pratt & Whitney unit makes jet engines.

Government auditors found cost overruns of about $1 billion. Although the Pentagon is absorbing the majority of the hit, Lockheed will still end up shouldering some of the burden. More importantly for United Technologies' future, however, is that continued overruns in the industry simply lend more evidence to those who want deeper defense cuts in an effort to trim the budget. United Technologies is better poised than some of its more defense-focused peers to weather such a storm, but it would still be a big hit to the conglomerate.

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