Access the Language Selection Menu and highlight "Menu". Arrow RIGHT. You should have just highlighted a 'Zippo lighter' which reveals some extra information on the film. This trick also works on the Spoken Languages and Captions & Subtitles submenu!
This easter egg was generously submitted/corrected by Satelk
From the main menu click on "Bonus Material", arrow left to advance to the next menu then click on "Cast & Filmmakers". From there advance to the 3rd menus and click on Steven Soderbergh. The first page of his filmograhy features a teaser trailer for The Underneath.
To activate the hidden triva you'll need to find 3 hidden Zippo lighters. Go to "Language Selection". From this menu highlight the main menu button and press your Right button to activate the hidden lighter. Next, go to "Spoken Languages" and also "Captions and Subtitles" and do the same thing for 2 other hidden lighters.
This easter egg was generously submitted/corrected by SirDonk