It seems the locals believe the excavation was for a missile silo (instead of for a missile-detection radar facility as it was)! Interesting how much misinformation is out there!

Captions that accompany each photo (on the donor web site):

Summer photo --
"... The first thing we checked out was the old missile silo. That`s right - the Boxford SF was to be a nuke site, but the funding was cut, and now it`s mostly state land. The old silo is just a pit, and it`s filled with water. ...."

Winter photo --
"... The structure here was originally set up as a missile silo. Boxford was going to be used a test site but lost funding. That might have been a misfortune at the time to some, but for us, we preferred the resulting consequence of having a great place to enjoy the outdoors. ...."

The following photo contributed by Mark Foster