Deep below the surface - the old Manhattan skyline rises over the sea floor, the Empire State Building still being recognisable at this point in history. A submarine explores the city, trying to solidify their understanding of the cataclysm that left their parents generation without land.
We know all of this for sure after studying the countless ancient cities at the bottom of the ocean.
This scene is very easy to picture in my head - it reminds me of that moment at the end of A.I. when the advanced future robots finds Hayley Joel frozen in the remains of the old city.