The Heart of Liberty City
There's a hidden chamber inside the Statue of Happiness that most players never find, and honestly it's not exactly signposted - there's literally a sign at the base that says 'No Hidden Content This Way,' which is Rockstar being cheeky. Steal a helicopter, land on the upper platform of the pedestal, and climb a ladder up through the statue's body. What's waiting for you is a huge, slowly beating heart wrapped in chains. Nobody's ever officially explained what it's supposed to mean, it's just there for the people determined enough to go looking.