How can the NYC subway flood if its underground and in tunnels?

Question

How can the NYC subway flood if its underground and in tunnels?

Answer 1

Same way a basement can flood-- if it's below the waterline and a leak develops, water will come in.

Answer 2

Actually a better question would be how can a subway NOT flood?

It is lower than surrounding streets and water will naturally want to accumulate in the tunnel.

They are constantlya fighting with elaborate drain systems and even drain pumps to keep unwanted water out of railroad and subway tunnels.