What is the farthest underground you can walk in NYC?

During a recent trip to Japan, I discovered you could walk essentially miles underground from one end of a station to the other, passing through supermarkets, walking paths, etc. It was wonderful to be completely shielded from rain, snow, and cold weather.

So I’m here to ask the knowledgeable wizards of r/nycrail: which two points in the NYC rail system are as far as possible and able to be traveled between while staying completely underground?

Let’s assume you can only walk in permitted areas (i.e. hopping the gate at the end of the subway platform and walking along a subway track to get between stations is not permitted).

EDIT: Bonus points if you can pinpoint a pair of 2 entrances on a map which meet the above criteria.