[The Australian Panther]

  I like the location and setting of The Octopus' lair, as a cave inside a hill dominated island inside a volcano's seawater-filled caldera, which can be approached and left unseen by using a submarine.

Robb, this is pretty much the same location used in the James Bond Movie, You Only Live Twice (1967).
The Hideout was built into the crater of an extinct Volcano on an isolated island. In that instance, the Volcano had a lid/door painted to look like water. It could be opened and entered by helicopter.
Used pretty much the same arrangement in the first Brosnan Bond, only this time the 'Volcano' was in South America.
Oh, And Robur, in Verne's 'Master of the World' - the second Robur novel, Robur builds his 'machine'

a ship , submarine, automobile , make it an airplane too they travel before vanishing ...at incredible speeds nothing can catch the phantom,

in a Volcanic Crater. 
Nothing new under the sun.
cheers!         

[The Australian Panther]

I am having problems posting reading group #271.
I have posted it but for some reason when I open CB+, it appears below #270. It's been up for 24 hours but doesn't appear in my 'show new replies to your posts' box. Posting to this group seems to work, so I am letting you know what's happening.
If you can see #271 on your screens, please post to it and let me know.
I have notified Mark.
Thank you.