This part of the External Tank is called the "LH2 Tank aft dome". There are really two large circular penetrations on it. They are the ones offset from the center of the tank.
One is the access hatch/manhole. (this description is from the LO2 tank part of the linked document. Further down it says the LH2 "manhole fitting was similar to those on both the LO2 tank domes and the LH2 tank forward dome")
A 45”-diameter manhole fitting, welded to the dome cap, provided a
36”-diameter clear access to the tank interior. The fitting
featured ninety-two attachment points for mating to the
machined aluminum manhole cover. The fifty-seven pound
manhole cover measured 40.32” in overall diameter and 0.185”
thick at the center. It provided an interfacing sealing
surface with the manhole fitting, as well as a means for making a
leak check of the primary seal.
The other is the "siphon plate hole". It's where this large device is mounted and was inserted during manufacture.
Here is an inside view of the aft part of the LH2 tank, manhole at the bottom, siphon at the top.
The 3rd circular feature in the center is an artifact of the manufacturing process and of the way the insulating foam was applied.
Here is a picture I took of an ET on the launch pad, stacked for flight. The manhole is at the top and the siphon plate is at the bottom.
Sources: HAER report, personal photo, External Tank - legacy of success
BONUS Something to think about: Why wasn't the siphon in the geometric center of the tank bottom? Mouseover for answer.
Because of the side mounting of the tank to the Orbiter with the engines mounted on the Orbiter, the acceleration vector was tilted away from the centerline of the tank.