Finding tunnels in Battery Park

[salemctu]

Where are the secret tunnels needed to find the ambrosia vaccine in Battery Park?

[DarkForge]

They're underneath Castle Clinton. You could either try fighting your way through the front entrance (follow Anna), or try the secret staircase behind the drinks machine, near to where you start. The locked box (so you'll need a lockpick) next to it contains the keypad. If you give the boy some food (I always use a candy bar), he'll give you the code.