That's the intertank - the cylinder that connected the bottom of the LO2 tank to the top of the LH2 tank.
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
This shows how the three sections fit together:
During the launch campaign for STS-133, some of these stringers caused a launch scrub - they were made from substandard material and cracked.