A small mine - part of the Rheidol United sett - with the least remains of the 5. The five are Erwtomau, Alltddu, Nantglas, Gwaithcoch and Foxpath. Alltddu deep adit lies by the track along the river and is masonry-lined. However, it's blocked about a fathom inside. This is the only place you'll see underground at Alltddu for the other adits - no. 1 and no. 2 (deep adit being no. 3) are blocked. No. 2 (shallow) was at the railway level and probably blocked during construction. Alltddu is linked with the rest of Rheidol United underground so it's possible to get to it via Erwtomau mine, but I havn't...yet!