Go to the front of the outbound Orange Line station at Chinatown and you'll see these mysterious tunnel entrances just past the end of the station, where the tracks curve on the way to Tufts Medical Center.
What strange wonders lie on the other side of this gateway?
Not much, alas. It's a few hundred feet of tunnel left over from when the Orange Line ran above ground south of Chinatown station, back before 1987, when the station was called Essex Street.
The new underground alignment, which runs along the turnpike, rather than crossing over it into the South End, and which included new stations at Back Bay and Tufts (then New England) Medical Center, required a new approach to the main Washington Street tunnel, a short distance to the west of the original "portal" at Ash and Nassau streets (you can watch a train enter the portal, around 9:30 in the video).
Chinatown station has other remnants of its days as Essex Street station, including a former exit now in place strictly for emergencies, also at the front of the southbound platform (note the bench for the weary):
There are a couple more emergency exits at the opposite end of the station. For years, two of them stood as these weird concrete boxes on the edges of a parking lot on Washington Street near Avery Street. The parking lot's been replaced by what will become luxury apartments, but the developers have incorporated the old structures into the building (if you're tall enough, you can peer in and see "Essex" in tile on the wall):