Pontrhydygroes (which translates as the bridge of the ford of the cross) is home to the longest drainage tunnel in mid Wales. It was constructed to drain four nearby lead mines, namely Logaulas, Penygist, Glogfach and Glogfawr.
The entrance to the main adit (level fawr in Welsh) is marked by an impressive portal of stone with a keystone bearing an inscription that reads: “THIS LEVEL WAS COMMENCED AD 1785”; the inscription also includes crossed miners’ picks.
The tunnel extends for around 1500 metres, where a small waterfall marks a change in level of around 7 metres. Silting shortly beyond this point prevents exploration of the full length of the tunnel.