Hermitage Hideout

Once again I spotted something blipworthy that I have run past on multiple occasions without noticing.

This little brick house  built into the river bank is on the North side of the hermitage. I'm guessing about 4ft high and inside was a spherical thing. Not sure what the use was, either military or mining or something else.  Interested if anybody does know what it is. I still don't know how I hadn't seen it before. 

I checked the orienteering map and it is mapped with a small black dot which could be misinterpreted as a boulder. Maybe the cave symbol is more appropriate. I've never seen it used as a control location, it would be a sneaky place to hide a flag.