Chambers JMZ (bmt nassau) - Underground river??

What's with the apparent underground river at the southern end of the Chambers JMZ station? They even put the tracks on mini trestles over the water.

Sorry from the crapola pics, but here you go:

<img src="http://static.flickr.com/110/292174676_3cdfc34891.jpg">

<img src="http://static.flickr.com/108/292174673_7f82a9b677.jpg">

Chambers (BMT) is notorious for being one of the worst stations in shape. (Practically) everything in it is messed up or ruined.

I agree, that is probably the worst looking station in all of Manhattan, it's unfortunately a huge mess. I'm curious about that "river" also now.