I understand that this trench is to be the insertion point for the drilling robot for the Central Subway. Every time I ride by it I get excited about the prospect like it's Christmas morning: "Is the new robot here yet? How about now? When do I get to see?"
Since this neighborhood is largely landfill, it will be going really deep (700ft, I think I read?) unlike the Transbay Tube which wasn't actually drilled: they lowered it from the surface.