The Last One

The alternate secret ending. Destroy all 13 orbs; the final one kills every light except the cornfield-bunker light - return to that bunker (the Field Research spot), enter the vault code (Up, Up, Right, Left, Right, Right, Right, Up, Up, Up, Right, Left, Left, Left), then in the final crawl space climb UP into the giant-orb chamber and interact to trigger the ending.