The "Tunnel Entrance Nr Goatchurch" is West Twin Brook Adit; the clue is in the name - the valley it is located in is known as West Twin Brook Valley since there is also an East Twin Brook Valley - see! - one's to the West and one's to the East and they are "twins" and also Brooks (i.e. streams), and valleys (no definition required, I hope!).
WTBA was dug out around the 1920s (IIRC) to intercept, hopefully considerable, natural springs sufficient to tap into a reliable fresh water source to supply human demand. The Adit (mined tunnel) goes horizontally into the hillside for about 300m (1,000ft); it begins very muddy with Lower Limestone Shale (slightly unconsolidated and loose in places) forming the walls and the further in you go, the further "back in geological time" you travel until you pass into the Old Red Sandstone layer - WTBA is pretty nearly only interesting for the geological sequencing which can be witnessed. There is a small runnel in the floor which collects small "leaks" and springs and feeds them towards the entrance where a settling tank inset into the floor feeds a pipe which emerges in the spring (the concrete square water fountain-thingy, near the bottom of the steps coming back down from Goatchurch Cavern). Visitors to WTBA disturb the settled mud and this results in the water coming out at the concrete square spring turning cloudy, hence proving that the water there is NOT guaranteed as safe to drink since any creatures gaining entry to WTBA and subsequently dying inside will pollute the water.
It did provide a constant source of water for many decades - although not in huge quantities - but the infamous floods of 1968 scoured out the bed of the brook including the piping (remains of which can still be seen to this day); the cost of the damage to the piping out-weighed the value of the water supply and hence was not restituted.
It is true that a few weeks ago the entrance was unsecure following yet more vandalism/forced entry resulting in the metal collar through which the chain/padlock was fixed being ripped out of its mounting; the Adit is once again secure with the same chain and padlock (combination) being reattached to something a tad more robust. The entrance will probably always be vandalised until it falls off its hinges, perhaps ironically squashing the perpetrators of said wanton destruction? .... who knows.
Parts of the roof are supported by love and imaginary stemples and being a mined passage it is possible that it could spontaneously collapse on a whim. Since it is not possible to find a mines inspector who would categorically state that it is "safe as houses" the best recommendation for any visitors is to say that an understanding of mines and the hazards associated with them is a good approach to have before considering a visit to this geologically interesting (but otherwise dull as ****) site.
