If you’ve ever read Heinlein’s “The Moon is a Harsh Mistress” you would know that the secret tunnel is an opening for launching rocks at the Earth via catapult...................
I read a couple of my Cousin Mike’s dirty books when I was a teenager.
Did this Heinlein also write The Curvaceous Candy Striper?
That was a good one.
Most of the mistresses in those books were too busy, well, doing the nasty dance to be launching rocks at earth.