Update..... Looks like campjohn64 hit it right on the nose - because that's where the entrance was....
Encouraged on by a friend, I decided to make a night trip to this missile site - and I found what I was looking for..... Here's the entrance:
The problem is that fucker is sealed shut - A metal plate is wielded in place, and short of bringing in a blow torch there is no way of me getting in there......