That's the intertank - the cylinder that connected the bottom of the LO2 tank to the top of the LH2 tank.

![enter link description here]][1]

It didn't contain propellant, but did contain the forward interface with the Solid Rocket Boosters, and was built for lightness and strength, with skin-stringer construction. The ribs you see were the stringers.

> The intertank is a steel / aluminum semimonocoque cylindrical
> structure with flanges on each end for joining the liquid oxygen and
> liquid hydrogen tanks. The intertank houses ET instrumentation
> components and provides an umbilical plate that interfaces with the
> ground facility arm for purge gas supply, hazardous gas detection and
> hydrogen gas boiloff during ground operations. It consists of
> mechanically joined skin, stringers and machined panels of aluminum
> alloy. The intertank is vented during flight. The intertank contains
> the forward SRB-ET attach thrust beam and fittings that distribute the
> SRB loads to the liquid oxygen and liquid hydrogen tanks. The
> intertank is 270 inches long, 331 inches in diameter and weighs 12,100
> pounds.

This shows how the three sections fit together:

![enter link description here]][2]

During the launch campaign for STS-133, some of these stringers caused a launch scrub - they were made from substandard material and cracked.

[![enter image description here][3]][3]
[![enter image description here][4]][4]

Sources

 - [NASA Press Manual][5]
 - [researchgate][6]
 - [stringer crack incident investigation report][7]


  [1]: https://science.ksc.nasa.gov/shuttle/technology/images/et-intertank_1.jpg
  [2]: https://www.researchgate.net/profile/Ivatury_Raju/publication/268479030/figure/fig1/AS:318207366844416@1452877956573/Basic-structural-elements-of-the-external-tank-left-to-right-LOX-tank-intertank-and.png
  [3]: https://i.sstatic.net/JfWjs.jpg
  [4]: https://i.sstatic.net/67UI5.jpg
  [5]: https://science.ksc.nasa.gov/shuttle/technology/sts-newsref/et.html
  [6]: https://www.researchgate.net/figure/Basic-structural-elements-of-the-external-tank-left-to-right-LOX-tank-intertank-and_fig1_268479030
  [7]: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20120014466.pdf