Sounds like Fish Cave:
Sunday 20th October 1991 Delayed by a couple of car boot sales but finally got to Prestatyn to look for Fish Cave. We parked in Brynllys West at NGR SJ 0648 8085, Brynllys East also leads up to the foot path. We walked across the fields and followed the a footpath to St Mary's Well at SJ 0670 8095. This is really a mine and you can see it by the spoil heap in the field beside the footpath. The mine consists of a rift, at the level of the footpath 2 feet wide and 10 -15 feet high. At 50 feet in it is 30 feet high and then the roof lowers abruptly and it 4 or 5 feet high. A mined passage a bit wet under foot with signs of flooding to 3 - 4 feet in depth because of the rubbish at the entrance. Up to 6 feet high and 4 feet wide at this point. Comes to a dead end after 100 feet, can see the entrance, adit is straight. Adit is in line with the last house along side the golf course.
We continued along the footpath and noticed a faint scree slope. This is below Fish Cave. The easy way is to follow the footpath to where they cross and then double back up the hill. The footpath passes the cave entrance and carries on up the hill to the Seven Sisters Cave [another mine']. There is very little left of the spoil heap in front of the cave from which the cave gets its name. It has been washed away long ago and is so overgrown it cannot be seen from the road. A small stream of water comes out of the cave and was at one time collected in a stone built basin. It is now in a state of disrepair. The entrance is 6 feet high and 3 feet wide with a muddy floor. It then decreases in height slightly. The mineralisation appears to be a horizontal vein about a quarter of an inch wide. After 150 feet the passage branches, can still see daylight, some stal on wall, main passage ends, a run in? a few feet back, another passage, still about 4 feet high, after 50 feet, flooded to a depth of 6 inches. A few yards back, an aven on right, 60 feet high, climbable, but not tried, strong draught noted. A couple of photos of Petra to show size of passage. Returned to entrance and followed footpath up to large grassy platform, old spoil heap. A better name would be Seven Dwarfs Caves as the entrance is so low, about 3 feet high by 3 feet wide and the same as far as I can see.
Cutting coal in my spare time.