Don't know if this has been broached, but smoke

[Igel]

5. There was already a bunker under the East Wing.

They've said that'll be upgrade/renovated, whatever that means.

There also has to be entry space and mechanical rooms, kitchen and planning/storage space, all the back-of-house 'stuff' for event centers.

The GRB Convention center in downtown Houston has a "ballroom" which is where large events happen. For rent to anybody--not part of a government facility--it's hosted dog and cat shows, vendor shows, housed thousands during emergencies (like Hurricane Harvey) and even hosted a national quilt show and, if I'm not mistaken, a comic con.