The "gazebo" is the Pulgas Water Temple near Woodside on the
peninsula. The underground picture is from the Sutter Gold Mine near
Sutter Creek on Highway 49. The company that gives tours of the mine
also runs several caves in the area. The pictures of the mine look
better for the purpose and the owners might be more willing to have
us work there since there are not natural formations to damage. The
best image that I saw was of the entrance to the Boyden Cavern which
has a big iron gate but it is in the middle of Kings Canyon National
Park which would be a long way to go. We would have to conceal or
remove the sign but with a suitable red light in the background this
could be made to look just like the gates of hell.