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.

    Photograph-like artists' renderings above and in the following pages show how the penthouse soars high over Monaco's skyline, providing stunning views from its rooms and terraces.