The life of a billionaire consists of many difficult decisions, one of which is deciding what to do with the piles of money. The world's richest people have diverse opinions on how to best spend their fortunes. From space travel to philanthropy, the pet projects of wealthy are a look into their personal interests and priorities. (For more information on the wealthy in Wall Street, see The 6 Highest Paid People On Wall Street.)
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1. Carlos Slim Helu
The world's richest man is worth $74 billion today. At 71, he is the chairman of the largest telecommunications and wireless carrier company in Latin America. He recently turned one of his passions into a civic gift for Mexico City, when he opened the Museo Soumaya, on March 28, 2011. The museum, which cost $70 million to construct, houses his private collection of 66,000 works of art, including works by Da Vinci, Toulouse-Lautrec, Picasso, Dalí, Rivera and Renoir. The museum is a tribute to his late wife Soumaya, who died of kidney failure in 1999. The President of Mexico and news anchor Larry King were among the guests on hand to host the preopening celebration of the museum.

2. Richard Branson
The wealthy thrill seeker, who is behind the more than 300 companies that make up megabrand Virgin, recently showed off a single-person submarine that he intends to use to explore the world's oceans. The submarine can also be chartered for $2,500 a day, perhaps to recoup some of the $17 million he spent to build it. He already has two dives planned for 2011. One to Mariana Trench, which at 36,000 feet is deeper than Mount Everest is high. The other trip is to Puerto Rico Trench at a depth of 28,000 feet. He hopes to break numerous Guinness Book World Records in his 18-foot long, carbon fiber and titanium craft. He also has plans to charter commercial flights into space through his company, Virgin Galactic. He also made headlines recently for his controversial plans to create an island sanctuary for lemurs.

3. Bill Gates
The Microsoft mogul gave up the position of richest man in the world to devote resources to his pet project. He has used $30 billion of his immense fortune to help people in the United States and in developing countries through The Bill & Melinda Gates Foundation. Gates and his wife serve as co-chairs of the organization, which also counts Warren Buffet as a trustee. The foundation has given out billions of dollars in grants to groups that support causes such as education, small farmers and public libraries.

4. Larry Ellison
The CEO and Director of a technology company called Oracle is the fifth-wealthiest person in the world, with a net worth of more than $39 billion. He is also an avid sailor and his pet project is his Oracle/BMW yacht, which he sailed to victory in 2010 in the America's Cup race. He has invested $100 million into participating in the race over the years, and was rewarded for his investment by becoming the first American to win the Cup since 1995. As winner, he earned the right to choose the competition's 2013 location. He picked San Francisco where he is a member of the Golden Gate Yacht Club.

5. Lakshmi Mittal
Indian billionaire Mittal owns Arcelor Mittal, the world's leading steel company. His net worth is $31.2 billion. He commissioned the ArcelorMittal Orbit for the 2012 Olympic Games in London. It will become the largest work of public art in the United Kingdom, and, at 400 feet tall, it is bigger than both the Statue of Liberty and Big Ben. Visitors will be allowed to climb the massive steel structure. The project costs 19.1 million British Pounds, and will be paid for both Mittal (16 million) and the London Development Agency (3.1 Million). (To read more on what the wealthy spend their money on, see 6 Outrageous Billionaire Purchases.)

The Bottom Line
The richest people in the world can literally fund anything that their hearts desire. These multi-million dollar pet projects serve as both status symbols and legacy for billionaires with unlimited bank accounts. (For steps on how you can become rich, check out 5 Easy Steps To Becoming A Millionaire.)