What exactly is the Mausoleum of the Arbiter?

The Prophet states that “Every Arbiter from first to last” is there.

Like, the dead bodies of the first couple? And like… What, just armor for the following? 

Bungie stated they had an idea of making Chief go through the Mausoleum in Halo 3 and fight against Flood Infected Arbiters, but that was scrapped. 

But still. What exactly is it? What’s in there? There’s 180+ “caskets” there.