Re: 911-foot-deep can be done . . .
It would involve blasting into the bedrock that makes the towering Manhatten skyline possible, but the pit would be very sturdy, and would not contribute to stress from nearby structures. The upper tiers of the pit could even be interlinked via pedestrian tunnels to other adjacent buildings.
The 911 feet, though, might be somewhat of a strain for many to hoof into and out of. A stylish outdoor elevator could be added as well.
Dan