Without knowing the sub soil structure, or seeing the engineering drawings, it looks like a fairly traditional type of raft construction since there was no mention of piling, where the foundation 'box'(in cross section) additionally provides for service ducts and storm drain functionality.
I would expect(but I don't know) that the required damping support - think earthquakes, will sit atop this below ground structure, for the above ground floors.
Tunnels?...no mystery. Services, cross site access( you can't expect everyone to walk the whole perimeter to see your buddy in another department or walk in the rain) and access to the central underground theatre from various building points. There will also be direct access to the theatre from outside the perimeter for joe public and (especially) the press - who wants them to all be traipsing through reception for presentations?
Apart from that, there's not much that can be said.
Wonder where the snoop-proof room is?