Interesting fact about C-5s: above the cargo compartment (which can carry two ABRAMS tanks, or 6 Greyhound busses) is the crew compartment foward of the wing, which includes kitchen, two three bunk crew rest areas, and a couple tables for card playing-- although that may have just been for maintanence.
Aft of the wing is 70+ seats for passengers above the crew compartment, plus the kitchen area. At the aft part of the kitchen area are two round holes, which are negative pressure doors. When the plane is on the ground (unpressureized) you cna swing them up into the plane and crawl though them into the 'hayloft' area. There are control cables and hydaulic lines, a platform you can climb, and then a ladder that goes up though the vertical stabilizer and then open a hatch from the inside. It drops down and then you can climb out on the top of the horizontal stabilizer. You wear a restraint harness and clip into holes and then walk around for inspections and such. the forrard part of the horizontal stab is called the bullet, and it goes 5-6 feet forward of the horizontal stab, and houses the jack screw. My first job as a mech for the USAF was to climb up there with a panel and a bag of fasteners since someone had forgot to put the access panel on in the hanger, and the wind was blowing too strongly to use a calivar, so with the planes moving around back and forth I had to straddle the bullet and slide forward to put this panel on with it shaking back and forth. Of course, they gave me no extra fasteners and it was 68 feet straight down if I dropped one. hella fun!
Anyway, the fact. The area aft of the pressure doors, where the platform and ladder was---that area is so far aft of the center of gravity that it was unusable space, so it was empty save control cables and such.
And the unused space is a larger volume than the whole cargo compartment on a C-130 Hercules.