The subways aren't just closed because of the risk of high winds—the threat of flooding in our subway tunnels is also quite serious. As MTA Chairman Joe Lhota put it at a briefing with Governor Cuomo today, "our subway system and salt water do not mix."

"Salt water can corrode switches quite easily" and could put the subway system's ability to work "in jeopardy," Lhota explained. And the MTA does have some very old (and not always well checked) switches. As such, the MTA is doing what it can to keep water out of the tunnels thanks to pumps and lots and lots of sand bags. In addition it has personnel at its five tunnels under the east river monitoring for flooding there. That personnel will remain "as long as it is safe."

So beyond the fear of salt water, how much water in general does it take to actually screw up the subway? Glad you asked! Back in 2007 a New York City Transit spokesman explained:

the subway tunnels were prone to flooding during heavy rains in the same way as the basements of many homes. Even without rain, groundwater routinely gets into a subway system and on a typical day with no rain, 13 million gallons of water are pumped out of the subway.

During very heavy rains, Mr. Fleuranges said, additional water pours into the system through grates in the sidewalk and through the subway entrances. Water then collects in the tunnels, and once the water reaches the rails, the trains cannot run.

That is because the rails contain electrical circuits that communicate with the signal system, indicating when a train is on a particular section of track. If the rails are covered with water, the signal system cannot work.

And if water reaches the third rail, which is 10 inches above the track bed, power must be shut off.

And with the storm surges going as high as they have today (and expected to go higher) you suddenly understand why the MTA was so keen to shut down yesterday.