The Pwll Du Tunnel was the longest horse-powered tramway tunnel to be built in Britain at 1,875 metres (6,152 ft) in length. It started in Blaenavon, Torfaen, Wales, and was originally a coal mine, running northward almost horizontally into a hillside. Later it was extended right through the hill and used to carry limestone from quarries at Pwll Du and Tyla to the ironworks at Blaenavon, and to carry pig iron from Blaenavon to the Garnddyrys Forge. The tramway was extended past Garnddyrys to Llanfoist Wharf on the Brecknock and Abergavenny Canal. The tramway from Pwll Du to the canal fell out of use when the railway came to Blaenavon and the Garnddyrys forge was closed in 1860, but the tunnel continued to be used to carry limestone to Blaenavon until 1926. It is now a scheduled monument and part of a UNESCO World Heritage Site.