Tunnel Hill can be found north of Owaka. It has some of the last remains of the old river railway running through The Catlins. Until 1971, it ran from Balclutha to north of Papatowai (Tahakopa), usually transporting timber.
The tunnel is the most southerly one of its kind in The World. It was built using handmade local bricks, and was completed back in 1895.
It is a short 10 minute walk; make sure you take a torch (you might even see glow worms).