I don't have one, but was thinking about your post today as I was getting a tour of a local sewage pump station (don't ask!). It is a small (maybe 8 x 10 foot) concrete building with a very secure metal door, but no windows. It has a big sign identifying it as a sewage pump station. Once inside, it went down three stories, and was huge inside.

So, that might be more than you are interested in, but what about a septic tank? Have one (or another one) installed, but don't attach it to the plumbing. I doubt that anyone would want to go inside the clearly labeled septic tank maintenance hatch, and you could store lots of stuff underground, in a cool, temperature controlled, dark environment.