A collapsed chamber beside Rt 3?

I don't know if I blogged about this. It is from Oct 2005:
:
I was walking between Rt 3 and Old Billerica Rd and came across what looked like a stone lined brook but, following it uphill a couple of yards, it turned out to be more like the entrance to a passage burial with a walled-off entrance, a collapsed ante-chamber and further deeper chambers also collapsed. It was something like this cutting into a knoll.The upper part of the picture shows the features seen from 3/4 view. To the left is the stone-lined passage which gets deeper uphill - to the right in the picture. The passage is obstructed by a wall and uphill/beyond that is a collapsed structure with a hole hinting at deeper recesses underneath. Still further uphill and in line with that, are several depressions indicating other collapsed spaces. The lower part of the picture shows a "bird's-eye" view of the surface features.

So I tried to take some pictures. Here is my first impression and a detail of the "collapsed hole".
Here are some of the "depressions" uphill above the passage and the collapsed hole.
In the second picture we are looking downhill with the bent over tree above the first collapsed hole.

Here are two views of the stone-lined passage. First shows the view up towards the wall and hole, the second shows the view back downhill.
This last picture, like the first, shows the relation between the wall and the collapsed hole.
I figure this is a chamber and ought to be recorded. Whatever it is, it is mighty peculiar. Perhaps someone will check it out more thoroughly someday. Backside of Rt 3 is not the sort of place you expect to find something interesting.