The Big Dig Tour / Tunnel Construction 7K. M. Peterson
Here's a last look at the construction site from the stairs leading out of the hole. I should have mentioned that the tunnel in this pictorial is not being bored, it's being constructed as a slurry-wall based "cut and cover" project. Part of the project is to allow street traffic to continue as normally as possible above the construction site, which is part of why there is a large structure just below ground level. It also affords sunlight in the tunnel at the early phases of construction before the "ceiling" is completed.