Silly question, what's holding the rooves up of those big spaces, is it thick concrete - or is it just very thick rock under NY ?
good question actually, isn't really clear to me either. I assume it's just the rock though, for a couple of reasons: looks like it in most pictures, the amount and strength of concrete needed to hold the weight of what's above is beyond imagination, last but not least: from the rock's point of view this tunnel isn't all that massive but rather a small hole through it's body that doesn't weaken it at all.