What's behind the Green Door?

When I was a nipper of 6 years of age there was a popular young person's musical piece being played over the wireless telegraphy machine called Green Door - the lyrics ran

"Don't know what they're doin'/ But they laugh a lot behind the green door/ Wish they'd let me in/ So I could find out what's behind the green door"

That really bugged me 'cause nobody could tell me what was going on, well half a century later I found this green door but I think I'd need a chain saw to find out what's behind it.

P.S. A quick Google for Green Door produces some interesting results, some best not viewed in the company of a maiden aunt with delicate sensibilities rendering her subject to an attack of the vapours at the mere sight of an uncovered table leg