Running beneath incline stage 3 (NPRN 91650) of the Llanfoist inclines (NPRN 405096) is a tunnel which allowed for the safe passage of people and animals when the incline was in use. This is one of two crossing points along the inclines, the other being a tramway bridge at the upper end of incline stage 2 (NPRN 270132).

The tunnel is 8.5m long by 1.8m wide and was at least 1.8m high. It is constructed of stone with straight side walls and a shallow symmetrical vault. The northern portal is still visible, and leads into a short 1.5m of the tunnel, running in a northwest direction before turning at right angles to run south under the incline. Only a small section of the stone arch of the southern portal is visible, the rest is buried beneath soil, stone and vegetation. The Llanfoist tithe map of 1844 shows two track ways running up to the incline at this point, which marked the boundary between woodland to the south, owned by Llanfoist Church and the enclosed, predominantly pasture land, to the north.

Incline stage 3 was surveyed by RCAHMW in December 2004 at the request of Cadw and World Heritage Site Partnership to provide an accurate and comprehensive archaeological record of the monument.

Louise Barker, RCAHMW, 18 October 2006.