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.
To activate the disc's hidden trivia, you must find three Zippo lighters. Go to "Language Selection" and highlight "Main Menu"; press the right arrow key to activate the concealed lighter. Then, go to the "Spoken Languages", and "Captions and Subtitles" sections; follow the same procedure to find the other hidden Zippos.
All easter eggs were generously submitted by SirDonk