WATCH: KSR tours The O2 in London, "The Greatest Venue in the World"

The O2 is considered by many the greatest entertainment venue in the world. The 20,000-seat arena is just one part of the massive complex underneath the Millennium Dome, which was built in 1999 to house an exhibition celebrating the turn of the third millennium. The arena itself was finished in 2007 and has hosted almost every major musical act you can think of, along with the ATP Tour Finals, NBA and NHL games, boxing matches, and events in the 2012 Summer Olympics. As of 2021, the O2 Arena was still the busiest music arena in the world in terms of ticket sales, beating out Madison Square Garden in New York City.