MIT's Great Dome is a double-layered structure; the bricks you see when you are underneath, in Barker Library, are not the bricks you see when you're walking on the roof. The area between the two layers is hollow. There's plenty of space for a human to walk or climb around, if said human doesn't mind breathing concrete dust.
In a neat architechtural trick, the inner layer is suspended from the outer. The Little Dome over building 7 is built the other way around, with the outer layer using the inner for structural support.
The top of the Great Dome used to be a skylight, which provided natural lighting for the library and nap space below. During World War II, that skylight was capped so as not to provide a big glowing target for the invading Nazi bombers. There's currently a project underway to reopen it, now that we don't have to worry about large explosions on US soil, or about people trying to blow up a university.