Heart in Statue of Happiness

Find a helicopter, which can easily be found around the city or at the airport. Fly out to the statue and hover near the top level of the base, jump out so that you land on the square rooftop which has four doors.
One of the doors has a sign which reads "No Hidden Content Through Here." These are filthy lies. Go through the door, it's an invisible entrance. Climb the ladder inside and marvel at the sight of the beating heart of Liberty City!

Shoot it, you know you want to. Blow it up! Nothing will stop it beating.