What's underneath us (or at least underneath Manhattan)?

Here’s an odd question: where is ground level in Manhattan, or more precisely in lower Manhattan?

I ask because I found myself the other day, in Tribeca (which is near the World Trade Center in lower Manhattan), looking down on a narrow space between a building and a street-level parking lot. It wouldn’t have been odd to look down there at a stairway coming up from a basement. But here what I was looking at, 10 or 15 feet below me, was ground, complete with a couple of trees growing out of it. It rather looked as if the street level where I was standing was not the true ground level at all, but rather the result of piling who knows what – concrete, pipes, power-lines, etc., etc., etc. – on top of the ground to reach where I now stood.

I tried to google this, and learned that Manhattan is built on granite that’s hard to tunnel down into. As a result, evidently, many Manhattan subway lines aren’t very far beneath the ground. But what I’m wondering about is roughly the opposite: how far above the ground has the city chosen to perch itself? I’m sure the answer is different in different parts of the city, but I begin to think that as a general matter the whole place has been built up several feet, at least, above the actual ground. New York’s skyscrapers are a remarkable sight, of course, but the idea that the bottom of the city, perhaps most of an entire island, might have been built up this much is quite remarkable too.