Whenever I drive along this road I think of a geocache I did years ago involving an old sandstone tunnel under the road. It has long been archived but today I set out to find the tunnel again. I did find it, and that cache has been revived as "Colo Vale Tunnel" nearby. But before rediscovering that one, I accidentally found this one! It is smaller and more overgrown but still a great example of historic sandstone engineering. The tunnel is usually dry so you can crouch-walk through it easily, but after heavy rain there could be some water in it.
I suspect there is a third tunnel further north, where Jumpdown Creek crosses the road, but that area is far too overgrown with lantana to explore.