Minllyn is a slate and slab quarry (nprn 310049) begun in the C19, the most productive phase of which began after the Carlyle Slate & Slab Company was formed in 1872. Work ceased in 1924. The tunnel is approximately 50m long WNW-ESE and took slate from the base of a quarry pit (nprn 527472), and represents the latest phase of working the pit. Water from the quarry now drains through the tunnel. The exit portal is 2m high and 2.5m wide, under a round arch of regularly-cut stone voussoirs.
Recorded as part of RCAHMW Uplands Initiative Project, W B Horton, H&H, 14/12/2012.