I was driving down a road today and stopped to wait for a construction crew while they were building an overpass. I noticed that beneath it they had installed a huge storm drain pipe and several others that led into it and one that led to a manhole to the surface. And most of it will be hidden behind a wall when its complete. Which made me think that a construction project like that would be perfect for hiding a cache of goods, equipment, a team, or anything else that needs to be hidden. The storm drains can be rerouted with pipes leaving the main really large pipe open and usable.