For Sale: The World's Most Expensive Apartment

One of the world's largest apartments is going up in one of world's smallest countries. It's a 35,500-square-foot, five-story penthouse being built on a tower in Monaco, a nation of less than a square mile.

The Sky Penthouse will also probably be the most expensive apartment ever built, expected to sell for nearly $400 million.

That's not surprising given how wealthy Monaco is -- nearly 30% of its 37,000 inhabitants are millionaires, according to WealthInsight.