In recent years, there's been a sharp increase in the number of $100 million (£76m) properties being built for the super-rich—just when we think we've seen the most extravagant billionaire mansion, another crops up that blows it out of the water. From opulent residences with nightclubs and recording studios to palatial private island retreats, these high-end homes are only available to the world's elite. Click or scroll to see inside these stunning billionaire mansions...