Yearby Icehouse
Visible from the main path through the woods at Yearby, although the tree cover made GPS reception tricky.
There appears to be a second chamber through the doorway at the rear, although i had neither a powerful torch or a ladder to venture any further to photograph it.

Some sort of ladder is needed if you want to get inside (or more importantly get back out afterwards)
There is a record of a hermit called Mick Plummer actually living here.