The Tunnel Loop is part of the Hauraki rail trail that starts back in Thames on the Coromandel Peninsula, although this section is an off shoot of the main rail trail as the trail goes to the Waihi gold mining village and you need to return back to the main trail that goes on to Te Aroha and trail end.
The walk section starts at the Crown Hill Road carpark this is the road that crosses the river with a foot bridge over the top and there is a carpark around one hundred meters up the road once you have crossed the river, the foot bridge is the start of the tunnel entrance the loop takes around one hour to complete.
The tunnel is powered by lights but they are not that bright and you need to watch for mountain bikes approaching from both directions, the tunnel was built in 1905 to transport the gold ore and building materials along the gorge, the tunnel is over one km long and lined with bricks the entire length.
Emerging out the other side you are directly under the main road and over the river all in one place, there is a great view up and down the river here, the loop return back to the Karangahake swing bridge in on the right hand side, straight ahead takes you through to Waihi, the return follows the river along a cliff face for around 2 kms before you arrive at the two swing bridges.
There are old gold mining structures and relics here to explore and also the junction of all the main walking tracks that make up the Crown Tramway walks, you can carry on to the Crown Hill car park along the river or go through the Scotsman Gully track which is around a 15 minute walk through a gully and a waterfall and emerges out at the top of Crown Hill road and around a ten minute walk down to the car park at the bottom.