More than 5,000 participants gathered on the chilly steps of the Sydney Opera House today for "Mardi Gras: The Base," an art installation by well-known artist Spencer Tunick. Subjects were instructed to do a number of poses, from standing up, lying down and even embracing cheek to cheek, for over an hour.

Tunick, who has produced almost 100 naked installations around the world, says, "the title of the work, commissioned by the Sydney Gay and Lesbian Mardi Gras, refers to the sameness of individuals, regardless of their sexual preferences."

This was his first Australian installation, following visits to Brazil, France, England, Austria and even the U.S., where Tunick has been arrested seven times for his work. Check out the photos -- who knows, among the bare rumps you might even catch Lady Gaga in disguise. (NSFW)