You can hear the difference
Between the boroughs
Riding the F train through the underwater stretch between East Broadway (Manhattan) and York Street (Brooklyn) always makes my ears pop a few times. Whenever I am coming home I feel an immense and inexplicable relief when the train leaves Manhattan (although I get antsy if I think too much about being in a tunnel underwater). I pretty sure the pop is the sound that the invisible underwater line separating the two boroughs makes when you cross it.