While not well-known, there was a fairly extensive mill at Biscuit City in Kingston, RI. Below the mill pond are a number of foundations and walls/channels. They go down quite a ways, ending with the large foundation of the original mill. I was about to call it a day when I noticed a large hole lined with stone. Upon closer inspection I discovered an underground room. It was about 20 by 8 feet with a 6 foot ceiling. Most of the stone was freestanding, but I found a few spots that had what appeared to be mortar in them. After 100 years or more, it crumbled like sand. My guess is that it was probably a root cellar. I'm just not sure what they would store in it at a mill.
Above the mill pond is the Great Spring, as the Indians called it. It still flows at a steady rate, and was the water supply for Kingston from 1901-1966. It is covered by a small building now, which is periodically broken into. The few times I went in I saw a water tank in the first room, and the second room contained the well, which was pretty much a hole in the ground in a dark room.
The entrance:
A cozy little place:
Detail of the stonework:
Artistic shot:
The building housing the spring: