Yes, it was framed, but rotted badly. Removed it, there is still a steel gerder from boulder to boulder along the top. It's about 4 ft deep in the pipe, with rocks and sediment at the bottom. There is no smaller pipe coming out from the mountain.
your new area is quite a storehouse of surprises and new things. Were you able to get a history on owners and usages of the property before you?
Yes, a bit...... The original owners had a single mobile home on the property for 35 years before they built this house. One of their parents lived below, this must explain the well down there, but that was awhile ago. There is also an old shed like structure, but their is a huge bees nest in there, so I won't mess with until fall or later. I'm anxious to see what's in it. I need to look at the septic designs again. I feel like I remember seeing something about a well down there.
The steel beam may have held a chain with a suspended pump or impeller hose to move water somewhere else. I'd dig it all out, line it with field stone and have a small spring fed swimming hole.