Investors need to do their own homework, but there are worse ways to learn about markets than by paying attention to the world's most successful money managers.

This week, hedge-fund legend and global philanthropist George Soros opined that the European Union is on the verge of collapse. Do other billionaire investors share his fear, and could that fear mean a global crash is on the horizon? Read on to find out what three of the planet's smartest investors are thinking.

Soros shatters markets
Warren Buffett may be the go-to buy-and-hold investor, but perhaps no single money manager has had as big of an impact on global markets as George Soros. Soros has been investing and profiting from ever-shifting market tides since the 1950s. His rapid-fire investment approach, passion for international affairs, and savvy market timing is the stuff of legend.

Although Soros has had many market successes during the past 60 years, his bet against the British Pound in the 1990s -- a move that broke the back of the Bank of England -- is a great example of Soros' uncanny ability to connect the dots between policy and profit. Unfortunately for investors, Soros' track-record makes his recent prognostication that the EU is on shaky footing even more dire.

In an interview with Gregor Peter Schmitz for the New York Review of Books, Soros said:

There is plenty to be nervous about. As she [Angela Merkel] correctly predicted, the EU is on the verge of collapse. The Greek crisis taught the European authorities the art of muddling through one crisis after another. This practice is popularly known as kicking the can down the road, although it would be more accurate to describe it as kicking a ball uphill so that it keeps rolling back down.

Soros went on to back up that image of futility by describing a slate of troubles facing the EU stretching from the never-ending Greek financial crisis to risks stemming from Eastern European leadership and Syrian migration. Given it's the EU, not the U.S., that's the biggest global economy, Soros' worry of economic doom in the region would seem to indicate he's similarly concerned about the EU's potential impact on American stock, currency, commodity, and bond markets, too.

Source: Jim Rogers Talks Markets.

Rogers' ramblings
Soros' former investment partner Jim Rogers may not work with Soros anymore, but he remains one of the biggest and most influential long-term global investors. Rogers is perhaps best known for embracing China early on in its economic expansion, and in an interview earlier this month on Bloomberg, Rogers said he was "worried about the world," citing Japan and the EU as major reasons for concern. 

Rogers went on to describe China as a victim of, rather than the cause of, global economic weakness. He also said that China, which has more debt on its books than before the Great Recession, is less of a concern to him than other more highly indebted parts of the world.

As a result, Rogers says he is short the U.S. stock market, short U.S. junk bonds, long the U.S. dollar, and long China, in spite of the country's short-term struggles.

Source: Vanguard Group

Bogle's bias
In the 1970s, Jack Bogle turned the investment universe on its ear by creating the modern index mutual fund at his now widely known Vanguard Group. Today, Vanguard has roughly $3 trillion (yes, trillion) dollars under management, the bulk of which are invested in index funds.

Unlike Soros and Rogers, both of whom have ostensibly shifted their investments to reflect their gloomy outlook, Bogle recommends that investors "do nothing." In an appearance on CNBC this week, he reminded investors that reacting to the market's inevitable machinations with decisions to curb monthly investments could result in investors paying a huge price in the long run.

That advice has merit given that the U.S. stock market has historically gone on to reach new highs after crashes, including in the wake of both the Internet Bust and the Great Recession.

Keeping perspective
Threats to global economies remain, and those threats could cause a slowdown in corporate profits that may lead to lower share prices. However, even Soros won't go as far as to claim that all is lost.

At his core, Soros is an optimist. After outlining the risks facing the EU, Soros also said:

I trained myself to look at the dark side. It has served me well in the financial markets and it is guiding me now in my political philanthropy. As long as I can find a winning strategy, however tenuous, I don't give up. In danger lies opportunity. It's always darkest before dawn.

Tempering talk of doom and gloom with a healthy dose of optimism has served investors well in the past, and that suggests that Bogle's advice may be the most worth following.