Litopia

A tunnel to the underworld, an opportunity to steal literary fire, to cheat the gods themselves!

Yes. Yes, you should open it

Sure. Evil was already released into the world. What could be worse?

I'm hearing this strange, ominous music in the background. . . do-do-do-do, do-do do-do . . . . HehHeh

A portal! Yey!

It's probably a "Priest Hole" where Catholic priests would hide during the Reformation.

Nothing in the survey presumably but an old mp might be telling. Recently found a 18C but possibly originally Medieval age lead mine shaft, not under the parent's house, but very close, under a cliff top chapel across the road. Six Cups and Chariot suggests your covered place could have been practical, to do with plumbing/water supplies/system. Or if nowhere near old or extant water infrastructure, remains of a former decorative structure, or else commemorative. A card of gardens, garden features, childhood, children, pets and nostalgia. There has been sorrow in the house, 6 Swords obsequies. But that's only to be expected in any older property. Anyway, not folk horror evillllllll. No Tower or DEVIL card.

Hoping not a mine shaft! Was wondering about a well, KE. There are a few other remains, incl a very atmospheric chimney, but so deep into the wood I can’t reach at the moment. Old waterworks were just up the road. We have people coming next week to break up the (new, badly laid) concrete yard but I won’t ask them to tackle this… It’s too old for that, needs treating with respect

A well, spring, pump or reservoir tank would seem feasible if the geology is suitable. If there no suitable scale map, a local dowser could maybe find out, short of disturbing it. Local councils have used dowsers in Teesdale, looking for old pipes. Have seen them in action in the fields behind the parent's place. The esoteric, alive and kicking in practical action.